electrochemical reduction of metal oxides in …electrochemical reduction of metal oxides in molten...

Electrochemical Reduction of Metal Oxides in

Molten Salts for Nuclear Reprocessing

Thesis submitted to University College London

by

Rema Abdulaziz

In fulfilment of the requirements for the degree of

Doctor of Philosophy

2016

ii

Declaration

I, Rema Abdulaziz, confirm that the work presented in this thesis is my own.

Where information has been derived from other sources, I confirm that this has

been indicated in the thesis.

Rema Abdulaziz

iii

To Professor Douglas Inman,

Abstract iv

Abstract

This thesis examines the electrochemical reduction of metal oxides in molten salts

for nuclear reprocessing applications. This is of particular importance for the

development of Gen IV nuclear power plants, where if pyroprocessing was to be

implemented, electro-reduction and refining of spent nuclear fuel are two main

steps in reprocessing. The objective of this research is to characterise and

understand the direct electrochemical reduction of UO2 to U metal in a LiCl-KCl

molten salt eutectic, as part of the nuclear pyroprocessing scheme, following a

similar approach to the FFC Cambridge Process for the reduction of TiO2 to Ti

metal. The voltammetric behaviour of reduction processes of metal oxides were

evaluated using electroanalytical techniques such as cyclic voltammetry and

chronoamperometry on different precursor types, such as thermally grown thin

oxide films, metallic cavity electrodes, and ‘a fluidised cathode’, a novel system

that was developed within this work. Material was characterised before and after

the electroanalytical experiments using scanning electron microscopy, X-ray

energy dispersive spectroscopy and X-ray diffraction.

Predominance phase diagrams, using recent thermodynamic data, for metal-molten

salt systems, relating the potential to the negative logarithm of the activity of O2-

ions (E-pO2-

), were produced for the range of spent nuclear materials (U, Pu, Np,

Am, Cm, Cs, Nd, Sm, Eu, Gd, Mo, Tc, Ru, Rh, Ag and Cd species), in both LiCl-

KCl at 500 °C and NaCl-KCl at 750 °C. The two salt eutectics were chosen as they

are the two main systems for pyroprocessing in the UK and US; temperatures were

selected within each salt’s normal operating range. All of the diagrams show

regions of stability for the different metal species, their oxides and chlorides at unit

activity; however, this activity can be altered in accordance with the equations

derived. Examples of selective electrochemical reduction are also demonstrated for

potential spent fuel reprocessing in both salt systems. In addition, the effects of

Abstract v

altering the operating temperature on the regions of stability within the diagrams

are also investigated.

The bulk of this research was on investigating the electrochemical reduction of

WO3 to W metal. Tungsten was selected as a chemical surrogate for uranium, due

to their similar physical and chemical properties, to investigate specific

electrochemical reduction and process parameters. Nonetheless, tungsten is an

important and desirable refractory metal because of its physical and chemical

properties, and the electrochemical route for producing it of high purity might

prove viable. The electrochemical reduction of WO3 to W metal has been assessed,

alongside a predominance diagram that was developed for the Li-K-W-O-Cl

system, and it likely to occur as following the reaction mechanisms: WO3 → WO2

→ W. A full reduction using the fluidised cathode process, with complete

conversion of the product to W was achieved, via applying a constant potential of -

2.14 V, with a high Faradaic efficiency. The reduction process is split into two

sections; the first where rapid reduction of WO3 occurs, the second where a slower

reduction of the remaining oxides in the product takes place. The deposited

material on the current collector is in the form of homogeneously distributed

particles.

Parameters, such as the metal oxide-salt ratio and the fluidisation rate, were

investigated, and depending on the desired means of recovery of the product, i.e. a

continuous flow retrieval or batch via removal of electrodes, these conditions can

be altered to suit. Particle coulometric analysis was also carried out, and an

electrochemical deposition model was also developed to estimate the porosity of

the deposit and the rate of its growth over time.

Finally, the electrochemical reduction of UO2 to U metal using the fluidised

cathode process was investigated. Voltammetry studies were conducted, and

alongside a predominance diagram that was constructed, the reaction path-way was

studied, and it is likely to take place following two different reaction mechanisms;

UO2 → U, via a 4-electron transfer step reaction; and UO2 → UO → U, via two 2-

electron step reactions. The route the reduction process follows depends on pO2-

Abstract vi

and potential, which is highly influenced by the type of metal oxide precursor used,

metallic cavity packed electrode or a fluidised cathode.

The main reduction potential using the fluidised cathode appeared to be -2.2 V. A

Faradaic current efficiency for the process was established, and found to be ~ 92%.

However this is a very rough estimate. The reduction process is split into three

sections; the first where a seeding process takes place at a low potential to allow

for the reduced uranium particles to be deposited onto the tungsten current

collector; the second where rapid reduction of UO2 particles takes place with a

growth in electrode size accompanied by an increase in current being passed; the

third where a slower reduction of the remaining oxides in the product occurs. As is

the case with the electrochemical reduction of tungsten oxide, the reduced product

can be collected from two areas; the deposit on the current collector’s surface and

the bottom of the reactor crucible.

The fluidised cathode is a robust, three-phase, high efficiency process. It was

studied here for the electrochemical reduction of WO3 and UO2, however, it is

likely applicable for other spent fuel oxides (such as UO3 and PuO2), and in the

production of refractory metals, such as titanium. Another proposed advantage of

using the fluidised cathode process is that it would eliminate certain steps in the

preparation of the precursor and recovery of the reduced metal in the FFC

Cambridge process with associated cost reductions.

Acknowledgements

During the many days and nights when this research was all consuming, several

persons provided indispensable support. People to whom I am particularly indebted

include:

Professor Douglas Inman, to whom this thesis is dedicated, for his insight,

advice and energy. He was an excellent mentor and friend, and I will always be

grateful to have known him.

My supervisor, Professor Dan Brett, for his unparalleled enthusiasm, guidance

and confidence in me. I thoroughly enjoyed our regular meetings and

discussions, and have learnt a lot from him.

My second supervisor, Dr. Paul Shearing, for his support and for giving me the

opportunities to gain experience working at Diamond Light Source and ISIS

Neutron Source.

The Department of Chemical Engineering, especially Professor Eva Sorensen,

Professor Eric Fraga, Dr. Simon Barrass and Professor Bill Maskell.

John Cowley and Michael Williams for their brilliant glassblowing skills, and

Martin Vickers for his help with XRD (UCL Department of Chemistry).

My friends and colleagues from the second floor, especially Shade, Matt,

Vassilis, Chara and Noor.

My friends and colleagues from the Electrochemical Innovation Lab,

especially Toby, Jay, Amal, Quentin, Ishanka, Ana and Rhod.

The EPSRC funded REFINE consortium, especially Dr. Clint Sharrad for

providing UO2 powder.

My parents, Ahmed and Naima, and siblings, Rabea, Aisha and Mariam, for

their unconditional love and support through the years.

My friends for being the best support system one could ask for, especially

Eoin, Holly, Rosalind, Fatma, Frances, Teodora, Malvika, Azza, Tom, John,

George, Clair, Chiaki, Roxy, Damir, Haydar, Tomek, Grace and Jessica.

Table of contents

Abstract .................................................................................................................... iv

Acknowledgements ................................................................................................. vii

Table of contents .................................................................................................... viii

List of Figures ......................................................................................................... xii

List of Tables ......................................................................................................... xix

Nomenclature .......................................................................................................... xx

1. Introduction ............................................................................................... 1

2. Background Review .................................................................................. 3

2.1 Nuclear fuel cycle ..................................................................................... 3

2.2 Nuclear reactors ........................................................................................ 4

2.3 Nuclear reprocessing ................................................................................. 6

2.3.1 Aqueous reprocessing ....................................................................... 7

2.3.2 Pyroprocessing .................................................................................. 9

2.4 Molten salts ............................................................................................. 11

2.4.1 Molten salt electrolysis ................................................................... 11

2.4.2 Advantages of using molten salts as electrolytes ............................ 12

2.4.3 Disadvantages of using molten salts as electrolytes ....................... 13

2.4.4 Operating temperatures of molten salts........................................... 13

2.4.5 Conductance of molten salts ........................................................... 15

2.4.6 Density of molten salts .................................................................... 15

2.5 Electrochemical reduction of metal oxides in molten salts ..................... 16

2.5.1 The Fray Farthing Chen (FFC) Cambridge process ........................ 16

Table of contents ix

2.5.2 The three-phase interline (3PI) ....................................................... 23

2.5.3 Challenges for electrochemical reduction in molten salts ............... 24

2.6 ‘Fluidised bed’ electrochemical processes .............................................. 26

2.7 Spent nuclear materials in molten salts ................................................... 29

2.8 Summary ................................................................................................. 33

3. Experimental ........................................................................................... 35

3.1 Electrochemical techniques ..................................................................... 35

3.1.1 Linear sweep and cyclic voltammetry............................................. 35

3.1.2 Chronoamperommetry .................................................................... 38

3.2 Material characterisation techniques ....................................................... 39

3.2.1 X-ray diffraction ............................................................................. 39

3.2.2 Scanning electron microscopy and energy dispersive X-ray

spectroscopy ................................................................................................... 40

3.2.3 Particle size analysis ....................................................................... 40

3.3 Experimental design ................................................................................ 41

3.3.1 Heating and temperature control ..................................................... 42

3.3.2 Electrochemical cell ........................................................................ 46

3.3.3 Electrodes ........................................................................................ 50

3.3.4 Electrolyte and cell atmosphere handling ....................................... 53

3.3.5 Electrochemical set-up .................................................................... 56

3.4 Summary ................................................................................................. 57

4. Predominance Diagrams ......................................................................... 58

4.1 Introduction ............................................................................................. 58

4.2 Theory ..................................................................................................... 61

4.3 Results ..................................................................................................... 68

4.4 Discussion ............................................................................................... 74

Table of contents x

4.4.1 Comparison of eutectics .................................................................. 74

4.4.2 Selective electro-reduction .............................................................. 74

4.4.3 Effect of temperature ...................................................................... 77

4.5 Conclusions ............................................................................................. 77

5. The Electrochemical Reduction of Tungsten Oxide ............................... 79

5.1 Introduction ............................................................................................. 79

5.2 Experimental ........................................................................................... 80

5.2.1 Apparatus ........................................................................................ 80

5.2.2 Chemicals ........................................................................................ 80

5.2.3 Procedure ........................................................................................ 82

5.3 Results and discussion ............................................................................ 82

5.3.1 Current efficiency ........................................................................... 93

5.3.2 Effects of metal oxide – salt ratio ................................................... 95

5.3.3 Effects of fluidisation rate ............................................................... 97

5.3.4 Particle coulometric analysis ........................................................ 100

5.3.5 Electrochemical deposition model ................................................ 103

5.4 Conclusions ........................................................................................... 106

6. The Electrochemical Reduction of Uranium Oxide .............................. 108

6.1 Introduction ........................................................................................... 108

6.2 Experimental ......................................................................................... 110

6.2.1 Apparatus ...................................................................................... 110

6.2.2 Chemicals ...................................................................................... 110

6.2.3 Procedure ...................................................................................... 111

6.3 Results and discussion .......................................................................... 112

6.3.1 Current efficiency ......................................................................... 119

6.4 Conclusions ........................................................................................... 122

Table of contents xi

7. Conclusions and Future Work ............................................................... 123

7.1 Conclusions ........................................................................................... 123

7.1.1 Predominance Diagrams ............................................................... 123

7.1.2 The electrochemical reduction of tungsten oxide ......................... 124

7.1.3 The electrochemical reduction of uranium oxide .......................... 125

7.2 Future work ........................................................................................... 126

7.2.1 Three dimensional microstructural analyses ................................. 126

7.2.2 Single particle reduction analyses ................................................. 126

7.2.3 ABBIS process .............................................................................. 127

7.2.4 Micro-electrode studies ................................................................. 129

7.2.5 TRISO fuel pyroprocessing .......................................................... 129

7.2.6 Electrochemical reduction of UO3, PuO2 and mixed oxide fuels .. 130

7.2.7 Electrochemical reduction of ThO2 ............................................... 130

7.2.8 Electrochemical reduction of TiO2 ................................................ 131

Dissemination ....................................................................................................... 133

Appendices ............................................................................................................ 136

Appendix A – calculations for immersion heater ................................................. 136

Appendix B – Electrode potential and pO2-

equations for predominance diagrams

of spent nuclear materials ..................................................................................... 138

Appendix C - Electrode potential and pO2-

equations for the Li-K-W-O-Cl system’s

predominance diagram .......................................................................................... 145

Appendix D - Electrode potential and pO2-

equations for the Li-K-Ti-O-Cl system’s

predominance diagram .......................................................................................... 146

References ............................................................................................................. 148

List of Figures xii

List of Figures

Figure 1.1 - World electricity generation by source of energy [5]. ........................... 2

Figure 2.1 - The nuclear fuel cycle. .......................................................................... 4

Figure 2.2 - Heat release from spent nuclear fuel [9]. .............................................. 5

Figure 2.3 - Solvent extraction for used nuclear fuel (UNF). ................................... 8

Figure 2.4 - Closed fuel cycles based on pyroprocessing developed at ANL (right

branch) and RIAR (left branch) [25]. ...................................................................... 10

Figure 2.5 - Liquid state domains (non-shaded zones) of salt mixtures [35]. ......... 14

Figure 2.6 - Specific conductance of pure molten salts vs. temperature [37]. ........ 15

Figure 2.7 - Density of molten salt eutectic mixtures vs. temperature [38]. ........... 16

Figure 2.8 - Calcium reduction of TiO2. (a) Calcium reduction. (b) Calcium

reduction in molten CaCl2. ...................................................................................... 18

Figure 2.9 - Cyclic voltammograms of titanium foils in molten CaCl2, scan rate: 10

mV s-1

, at 800 ˚C. (a) Oxide-scale-coated titanium foil. (b) As received titanium

foil [39]. .................................................................................................................. 19

Figure 2.10 - Electrolytic cells for the reduction of TiO2 pellets via the FFC

Cambridge Process [39]. ......................................................................................... 19

Figure 2.11 - Cyclic voltammograms of TiO2 and inert electrode in CaCl2, scan

rate: 50 mV s-1

, at 900 ˚C [49]. ............................................................................... 21

Figure 2.12 - Stages of the FFC Cambridge process [60]. ...................................... 22

Figure 2.13 - Schematic representation of a three-phase interline (3PI) connecting a

solid metal phase, a solid compound phase (metal oxide), and a liquid electrolyte

phase (molten salt). The grey planes are the interlines between two neighbouring

phases. At the 3PI, electron transfer occurs between the metal and the metal oxide

and oxygen anion transfer between the metal oxide and the molten salt. ............... 23

Figure 2.14 - Schematic of the 3PI propagation mechanism of a cylindrical metal

oxide pellet. (a) Propagating from the current collector along the surface. (b)

Propagating within the pellet [61]. .......................................................................... 25

List of Figures xiii

Figure 2.15 - Fluidised bed electrolysis reactor according to Fleischmann and

Goodridge [95]. ....................................................................................................... 27

Figure 2.16 - Schematic of a fluidised cathode process in a molten salt showing

various reaction mechanisms between particles in the melt and the electrode. ...... 28

Figure 2.17 – Schematic of the electrorefining process developed by the ANL,

operated at 500 °C, where AM = alkali metals, AEM = alkaline earth metals, MA =

Np, Am and Cm, NM = noble metals, RE = rare earth metals, FP = fission product

[104]. ....................................................................................................................... 30

Figure 3.1 - (a) Potential vs. time waveform for linear sweep voltammetry (LSV).

(b) Potential vs. time waveform for cyclic voltammetry (CV). (c) Current vs. time

response corresponding to LSV. (d) Current vs. potential response corresponding to

CV. .......................................................................................................................... 36

Figure 3.2 - Oxidised species (O) concentration vs. distance at different times, 1-5,

during a linear sweep voltammetry. ........................................................................ 37

Figure 3.3 - Current vs. time response for chronoamperommetry. ......................... 38

Figure 3.4 - X-ray diffraction schematic of a crystal. ............................................. 40

Figure 3.5 - Perspex dry glove-box containing molten salt electrochemical cell. .. 42

Figure 3.6 - Schematic of the rig set-up, showing components inside and outside

the dry glove-box. ................................................................................................... 43

Figure 3.7 - Phase diagram for NaNO3-KNO3 eutectic [120]. ................................ 44

Figure 3.8 - Dimensions of silica tubing for immersion heater. ............................. 44

Figure 3.9 - Schematic for heating apparatus. ........................................................ 45

Figure 3.10 - Immersion heater rig. ........................................................................ 46

Figure 3.11 - Dimensions of cell envelope. ............................................................ 47

Figure 3.12 - Electrochemical cell schematic. ........................................................ 48

Figure 3.13 - Electrolytic cells. (a) For a typical cell set-up for the electrochemical

reduction of thin films or metal cavity electrodes (MCEs), (b) for the

electrochemical reduction of metal oxide powder using the fluidised cathode

method. ................................................................................................................... 49

Figure 3.14 - Ceramic cell heads hole arrangements. (a) For a typical cell set-up.

(b) For a fluidised cathode set-up. Where, 1 is the argon inlet, 2 is the argon outlet,

List of Figures xiv

3 is for the reference electrode, 4 is for the working electrode or current collector

(also used for the sacrificial electrode in pre-electrolysis), 5 is for the

thermocouple, 6a for the anode, and 6b for the anode compartment containing the

anode. ...................................................................................................................... 50

Figure 3.15 - Cross section of the Ag/Ag+ reference electrode............................... 51

Figure 3.16 – (a) Working electrode for fluidised cathode set-up. (b) Metallic

cavity working electrode (MCE)............................................................................. 53

Figure 3.17 - Cyclic voltammogram showing the potential window of LiCl-KCl

eutectic at 450 °C, scan rate 50 mV s-1

, and reference electrode: Ag/Ag+. ............. 55

Figure 3.18 - Chronoamperogram showing a typical pre-electrolysis of LiCl-KCl

eutectic at 450 °C, set voltage: -2.300 V, and reference electrode: Ag/Ag+. .......... 55

Figure 3.19 - X-ray diffraction spectrum (Mo Kα) of treated LiCl-KCl salt,

showing KCl (165593-ICSD [130]), and LiCl (26909-ICSD [131]). ..................... 56

Figure 4.1 - Example of the three main regions of stability in a predominance

diagram (based on a LiCl-KCl salt eutectic) and showing the nature of reactions

leading to horizontal, vertical and diagonal lines.................................................... 63

Figure 4.2 - Predominance diagram for the Li-K-O-Cl system at 500 °C,

illustrating the relationship between oxygen pressure, oxide activity and potential,

E, relative to the standard chlorine electrode. ......................................................... 65

Figure 4.3 - Predominance diagrams of (a) U, (c) Pu, (e) Np, and (g) Am in LiCl-

KCl at 500 °C. (b) U, (d) Pu, (f) Np, and Am are in NaCl-KCl at 750 °C. ............ 70

Figure 4.4 - Predominance diagrams of (a) Cm, (c) Cs, (e) Nd, and (g) Sm in LiCl-

KCl at 500 °C. (b) Cm, (d) Cs, (f) Nd, and Sm are in NaCl-KCl at 750 °C. .......... 71

Figure 4.5 - Predominance diagrams of (a) Eu, (c) Gd, (e) Mo, and (g) Tc in LiCl-

KCl at 500 °C. (b) Eu, (d) Gd, (f) Mo, and Tc are in NaCl-KCl at 750 °C. ........... 72

Figure 4.6 - Predominance diagrams of (a) Ru, (c) Rh, (e) Ag, and (g) Cd in LiCl-

KCl at 500 °C. (b) Ru, (d) Rh, (f) Ag, and Cd are in NaCl-KCl at 750 °C. ........... 73

Figure 4.7 - Predominance diagrams of U and Pu species. (a) in LiCl-KCl at 500

°C, (b) in NaCl-KCl at 750 °C. ............................................................................... 76

Figure 4.8 - Predominance diagrams of Am and Cm species. (a) in LiCl-KCl at 500

°C, (b) in NaCl-KCl at 750 °C. ............................................................................... 76

List of Figures xv

Figure 4.9 - Predominance diagram for the Li-K-U-O-Cl system at 500 °C. ......... 78

Figure 4.10 - Predominance diagram for the Li-K-U-O-Cl system at 800 °C. ....... 78

Figure 5.1 - Average particle size distribution of WO3 powder in terms of volume

percentage, also showing the maximum size distribution and the minimum, with a

standard deviation of 2. Mean particle diameter: 30.92 μm, median particle

diameter: 29.54 μm. ................................................................................................ 81

Figure 5.2 - X-ray diffraction spectrum (Mo Kα) of as-received WO3 powder

sample, showing WO3 (1620-ICSD [173]). ............................................................ 82

Figure 5.3 - Predominance diagram for the Li-K-W-O-Cl system at 500 °C. ........ 84

Figure 5.4 - (a) Cyclic voltammogram of WO3 thin film cathode in LiCl-KCl

eutectic at 450 °C, scan rate: 50 mV s-1

, reference electrode: Ag/Ag+. (b) Cyclic

voltammogram of WO3 fluidised cathode in LiCl-KCl eutectic at 450 °C, scan rate

50 mV s-1

, reference electrode: Ag/Ag+. ................................................................. 86

Figure 5.5 - Cyclic voltammogram of WO3 fluidised cathode in LiCl-KCl eutectic

at 450 ˚C, scan rate: 50 mV s-1

, and reference electrode: Ag/Ag+. ......................... 87

Figure 5.6 - Chronoamperogram of WO3 fluidised cathode in LiCl-KCl eutectic at

450 °C, 40 g WO3, argon flow rate: 800 cm3 min

-1, set voltage: -2.14 V, reference

electrode Ag/Ag+. 1 - 4: diagrammatic representation showing the evolution of the

cathode's surface area over time. ............................................................................ 88

Figure 5.7 - (a) Photographs of current collector before the reduction process, 1,

and after, 2, (b) photograph of the solidified melt showing separate layers of WO3

and W. ..................................................................................................................... 89

Figure 5.8 - (a) EDS spectrum of the deposit on the current collector's surface. (b)

Photograph of the working electrode after the chronoamperometry showing the

deposited W. (c) SEM image of the cross-section of the solid W working electrode,

2, and the deposit grown, 1. .................................................................................... 90

Figure 5.9 – SEM image of the as-received WO3 particles, and (b) product W

obtained from the solidified melt as a separate layer to the LiCl-KCl. EDS spectra

(penetration depth ~6 μm) showed the stoichiometric composition of WO3 in (a),

and the absence of oxygen in (b). ........................................................................... 91

List of Figures xvi

Figure 5.10 - SEM image of the cross-section of the current collector showing the

deposited tungsten. .................................................................................................. 92

Figure 5.11 - SEM images of the current collector and deposit, at different

magnitudes, showing the homogeneity of reduced particles. ................................. 92


450 ˚C, 4 g WO3, argon flow rate: 800 cm3 min

-1, reference electrode: Ag/Ag

+, set

voltage: -2.135 V. ................................................................................................... 94

Figure 5.13 – X-ray diffraction spectrum (Mo Kα) of sample of product after

complete reduction, showing W (52344-ICSD [176]), and KCl (165593-ICSD

[130]). ..................................................................................................................... 95

Figure 5.14 - Chronoamperograms of WO3 fluidised cathode in LiCl-KCl eutectic

at 450 ˚C, argon flow rate: 800 cm3 min


+, set voltage:

-2.14 V, for different weight percentages of WO3 in the melt, as indicated. .......... 96

Figure 5.15 - Count of charge per spike relative to a normalised line of best fit, at

different WO3 – LiCl-KCl eutectic ratios. Where, (a) is at 0.312 wt% WO3, (b)

20.00 wt%, and (c) 27.27 wt%. ............................................................................... 97


450 ˚C, 40 g WO3, reference electrode: Ag/Ag+, set voltage: -2.14 V, showing the

effect of different argon flow rates bubbled through the melt. ............................... 99

Figure 5.17 - Count of charge per spike relative to normalised line of best fit, at

different argon flow rates. Where, (a) is at 200 cm3 min

-1, (b) 400 cm

3 min

-1, (c)

600 cm3 min

-1, (d) 800 cm

3 min

-1, (e) 1000 cm

3 min

-1, (f) 1200 cm

3 min

-1, (g) 1400

cm3 min

-1, (h) 1600 cm

3 min

-1, and (i) 1800 cm

3 min

-1. .......................................... 99

Figure 5.18 - (a) Chronoamperogram of WO3 fluidised cathode in LiCl-KCl

eutectic at 450 ˚C, 40 g WO3, reference electrode: Ag/Ag+, set voltage: -2.14 V, (b)

zoom-in of a section containing 200 spikes used for coulometric analysis. ......... 101

Figure 5.19 - (a) Relative frequency of spike durations in Figure 5.18 (b) from

coulometric analysis. (b) Count of charge per spike from coulometric analysis. . 101

Figure 5.20 - (a) Count of radii sizes of WO3 reactant particles from particle

coulommetry analysis. (b) SEM image of a large WO3 particle. .......................... 102

Figure 5.21 – Schematic showing parameters of the current collector and the

deposit used in the electrochemical deposition model. ......................................... 103

List of Figures xvii




+, set

voltage: -2.14 V. ................................................................................................... 105

Figure 5.23 – Deposit growth on current collector over time, predicted via

electrochemical deposition model. ........................................................................ 105

Figure 6.1 – Conceptual flowsheet for the treatment of used LWR fuel [181]. .... 109

Figure 6.2 - X-ray diffraction spectrum (Mo Kα) of as-received UO2 powder

sample, showing UO2 (35204-ICSD [182]). ......................................................... 111

Figure 6.3 - Cyclic voltammogram of UO2 in molybdenum metallic cavity

electrode in LiCl-KCl eutectic at 450 ˚C, scan rate: 20 mV s-1

, reference electrode:

Ag/Ag+. ................................................................................................................. 114

Figure 6.4 - Cyclic voltammograms of UO2 in molybdenum metallic cavity

electrode, at different scan rates, in LiCl-KCl eutectic at 450 ˚C, reference

electrode: Ag/Ag+ (A newly packed MCE was used for each scan). .................... 114

Figure 6.5 – Cyclic voltammetry of UO2 fluidised cathode in LiCl-KCl eutectic at

450 °C, 5 g UO2, argon flow rate: 600 cm3 min


+. .. 115

Figure 6.6 - Predominance diagram for the Li-K-U-O-Cl system at 500 °C,

showing the reaction pathway for the reduction of UO2 to U in LiCl-KCl using a

fluidised cathode process and a metallic cavity electrode (MCE). ....................... 116

Figure 6.7 - Chronoamperogram of UO2 fluidised cathode in LiCl-KCl eutectic at

450 °C, 5 g UO2, argon flow rate: 600 cm3 min

-1, set voltage: -2.2 V, reference

electrode Ag/Ag+. .................................................................................................. 118

Figure 6.8 – (a) Photograph of reduced uranium deposited on tungsten working

electrode, (b) photograph of the cross-section of the uranium product at the bottom

of the crucible, (c) photograph of the solidified product, showing two separate

layers of salt and uranium metal. .......................................................................... 118

Figure 6.9 – SEM images of (a) the as-received UO2 particles, and (b) product U

obtained from the solidified melt as a separate layer to the LiCl-KCl. ................. 119


450 ˚C, 4 g UO2, argon flow rate: 600 cm3 min


+, set

voltage: -2.2 V. ..................................................................................................... 121

List of Figures xviii

Figure 6.11 - X-ray diffraction spectrum (Mo Kα) of sample of product after

complete reduction, showing peaks for α-U (43419-ICSD [187]), and KCl

(165593-ICSD [130]). ........................................................................................... 121

Figure 7.1 – Experimental set-up for single particle reduction studies. ................ 127

Figure 7.2 – Schematic for ABBIS process (with LiCl-KCl eutectic as an example).

.............................................................................................................................. 128

Figure 7.3 – Illustrative cutaway drawing of a TRISO fuel particle [198]. .......... 129

Figure 7.4 - Predominance diagram for the Li-K-Ti-O-Cl system at 500 °C. ...... 132

Figure 7.5 - Cyclic voltammogram of TiO2 thin film cathode in LiCl-KCl eutectic

at 450 °C, scan rate: 50 mV s-1

, reference electrode: Ag/Ag+. .............................. 132

List of Tables xix

List of Tables

Table 2.1 - World operational nuclear reactors [8]. .................................................. 5

Table 2.2 - World commercial reprocessing capacity [12]. ...................................... 7

Table 2.3 - Melting temperatures of salt mixtures and pure salt constituents [31,

35]. .......................................................................................................................... 14

Table 2.4 – Redox potentials of relevant elements in LiCl-KCl eutectic salt at

indicated operating temperatures (V vs. Ag/Ag+ reference electrode) [104-107]... 31

Table 2.5 – Generation IV reactor designs under development [108]. ................... 32

Table 3.1 – Guideline for voltage required to achieve different temperatures of 3 kg

of NaNO3-KNO3 thermostatic salt bath after 1.5 h. ................................................ 45

Table 4.1 - List of nuclides commonly considered in burn-up credit criticality

analyses (from NEA 2011) [145] and used as the basis for the systems examined

here. ......................................................................................................................... 60

Table 4.2 - Gibbs energy of formation for nuclear materials at 500 °C and 750 °C.

................................................................................................................................ 66

Table 5.1 - Gibbs energy of formation at 500 °C for species in the Li-K-W-O-Cl

system. .................................................................................................................... 83

Table 5.2 - Thermodynamically calculated values of pO2-

and potentials required

for Equations 5.1, 5.2, 5.6 and 5.7 to take place. .................................................... 84


and potentials required

for Equations 6.1 and 6.2 to take place. ................................................................ 113

Table 7.1 - Voltage, power and heat needed to reach specified temperatures of the

thermostatic salt bath. ........................................................................................... 137

Nomenclature xx

Nomenclature

List of abbreviations

3PI Three phase interline

AEM Alkaline earth metal

AGR Advanced gas-cooled reactor

AM Alkali metal

ANL Argonne National Laboratory

BWR Boiling water reactor

CV Cyclic voltammetry

EDS X-ray energy dispersive spectroscopy

EXAm Extraction of americium

FBR Fast breeder reactor

FFC Fray Farthing Chen

FIB Focused ion beam

FP Fission products

GCR Gas-cooled, graphite-moderated reactor

GFR Gas-cooled fast reactor

GHG Greenhouse gases

HLW High level waste

IFR Integral fast reactor

LEP Liquid electrolyte phase

LFR Lead-cooled fast reactor

LILW Low and intermediate level waste

LSV Linear sweep voltammetry

LWGR Light-water-cooled, graphite-moderated reactor

LWR Light water reactor

M Metal

MA Minor actinide

MCE Metallic cavity electrode

MOX Metal oxide

MSR Molten salt reactor

MTHM Metric tonne of heavy metal

NEA Nuclear Energy Agency

NM Noble metal

O Oxidised species

OS Ono Suzuki

Nomenclature xxi

PHWR Pressurised heavy-water-cooled and moderated reactor

PUREX Plutonium-uranium extraction

PWR Pressurised water reactor

R Reduced species

RE Rare earth metal

RIAR Research Institute of Atomic Reactors

SANEX Selective actinide extraction

S.Cl.E Standard chlorine electrode

SCP Solid compound phase

SCWR Supercritical-water-cooled reactor

SEM Scanning electron microscopy

SFR Sodium-cooled fast reactor

SMP Solid metal phase

TBP Tri-butyl phosphate

TRISO Tristructural-isotropic fuel

TRU Transuranic material

TRUEX Transuranic extraction

UNF Used nuclear fuel

UOX Uranium oxide

VHTR Very-high-temperature reactor

XRD X-ray diffraction

List of Symbols

ΔG0 Gibbs energy of formation J mol

-1

ΔT Change in temperature K

θ Angle Rad

λ Wavelength m

ρ Density kg m-3

a Thermodynamic activity -

C Concentration mol m-3

CO Concentration of O mol m-3

Cbulk Concentration of bulk mol m-3

c Specific heat capacity J kg-1

K-1

DO Diffusion coefficient of O m2 s

-1

d Interplanar spacing m

E Potential kg m2 s

-3 A

-1 (V)

E0

Standard electrode potential kg m2 s

-3 A

-1 (V)

Ed Defined potential kg m2 s

-3 A

-1 (V)

EO/R Reduction potential for redox couple O and R kg m2 s

-3 A

-1 (V)

e Electronic charge (1.602 × 10-19

C) A s (C)

Nomenclature xxii

F Faraday constant (96485 C mol-1

) A s mol-1

(C mol-1

)

I Current A

i Current A

ID Inner diameter m

k Specific conductance A2 s

3 kg

-1 m

-3 (S m

-1)

L Length m

L Latent heat J kg-1

M Molar mass kg mol-1

m Mass kg

N Number of reduced oxide units -

NA Avogadro constant (6.022 × 1023

mol-1

) mol-1

N(t) Molar accumulation at t mol

n Number of moles mol

OD Outer diameter m

P Partial pressure kg m-1

s-2

(Pa)

P Power J s-1

pO2-

-log10[O2-

] -

Q Charge A s (C)

Q Heat J

q Flux mol m-2

s-1

R Universal gas constant (8.314 J K-1

mol-1

) J K-1

mol-1

R Resistance kg m2 s

-3 A

-2 (Ω)

r Radius m

r0 Radius at t = 0 m

r(t) Radius at t m

T Temperature K

Tf Final temperature K

Ts Start temperature K

t Time s

ts Start time s

td Defined time s

V Voltage kg m2 s

-3 A

-1 (V)

V(t) Volume at t m3

x Distance m

z Number of e transferred per reduced formula -

unit of oxide

1. Introduction 1

1. Introduction

The world’s energy consumption increases with the growth of economies, and

demand is expected to increase by a factor of 1.5-3 by 2050. Furthermore,

standards of living are directly related to the availability of electricity, for which

the demand is expected to be twice as large by 2050 as well [1]. The majority of

global electricity is derived from the burning of fossil fuels, which produces

greenhouse gases (GHG) that are large contributors to global climate change.

Though the use of renewable energy resources continues to develop, it is still not

satisfactory for fulfilling energy demands [2]. Figure 1.1 shows the breakdown of

the world electricity generation by source of energy, in 1971 and 2010. Nuclear

energy for electricity generation offers large-scale low-carbon energy production,

but suffers with challenges such as the handling of radioactive waste materials.

For low and intermediate level waste (LILW) disposal facilities already exist in

twenty three countries around the world. As for high level waste (HLW) no such

facilities exist. However, there are storage facilities in operation. The disposal of

HLW is the only step in the civilian nuclear fission cycle with no large scale

facilities in operation yet [3].

The reprocessing of spent nuclear fuel is important as it can reduce the amount of

waste to be disposed of, and lessens the indirect environmental footprint. Most of

the GHG emissions associated with nuclear power come from construction (40%),

mining activities (32%) and enrichment (12%); whereas, conversion, disposal and

reprocessing are only responsible for 5, 2 and 7% respectively [4]. Therefore, by

reducing or forfeiting the mining and the enrichment steps, via reprocessing, the

GHG emissions would be significantly reduced.

While the reprocessing of some of the spent fuel materials increases the efficiency

of the fuel-cycle, the reprocessing of other materials, such as the minor actinides, is

only important for the public acceptance of civilian nuclear power. Also, the more

1. Introduction 2

materials are reprocessed, the less space will be needed for underground storage in

the future.

Figure 1.1 - World electricity generation by source of energy [5].

High temperature molten salt reprocessing technology (pyroprocessing) offers a

range of advantages when compared to aqueous reprocessing techniques. This is

due to the fact that the technology is inherently ‘safe’ as it is resistant to

proliferation and does not provide an environment for criticality accidents to occur;

it also utilises compact easily accessible facilities.

The aim of this research is to develop the understanding of spent nuclear materials

behaviour in molten salts, via thermodynamic and electrochemical techniques, and

to investigate electrochemical reduction processes from oxides to metals, which

can prove to be significant in future generation IV nuclear power plants. New

reactor designs are investigated as well, to improve the efficiency of such

processes.

This thesis consists of a literature review where a concise background of nuclear

pyroprocessing, molten salts, electrochemical reduction, and spent nuclear

materials in molten salts is given. This is followed by an experimental Chapter

where a background of the electrochemical techniques and characterisation

techniques is included, in addition to specific experimental designs used later in

this work. A theoretical Chapter where predominance diagrams, summarising the

thermodynamic and electrochemical behaviour of spent nuclear materials in molten

salts, were produced is also included. Then, two experimental Chapters were the

electrochemical reduction of WO3 and UO2 were investigated. Finally, this is all

summarised in the conclusion, where also suggestions for future work were

provided.

2. Background Review 3

2. Background Review

This Chapter aims to present a critical review of the relevant literature and has been

split into sections that cover: a background review on nuclear energy and

reprocessing technologies, molten salts and their applications, electrochemical

reduction processes for metals production in molten salts, fluidised bed electrode

technologies, and the electrochemistry of spent nuclear materials in molten salts

found in the literature.

2.1 Nuclear fuel cycle

The closed nuclear fuel cycle is outlined in Figure 2.1 and comprises of the

following main steps:

1. Mining and milling of the ore (the creation of the ‘yellow cake’).

2. Purifying the ore and enriching of the fissile U235

content, if needed,

then manufacturing the fuel.

3. Utilising the fuel in various reactor types, normally as fuel pellets in

fuel rods. This is different for different reactors.

4. Reprocessing of the spent fuel at the end of its life time, where

separating and recycling of uranium and plutonium occurs.

5. Safe and secure disposal of the remaining radioactive waste.

To close the back-end of the nuclear fuel cycle, uranium and plutonium are

reprocessed, then recycled back into fuel for reactors. These reprocessing steps are

highlighted in red in Figure 2.1. This is called the ‘twice through cycle’. Ignoring

reprocessing and the direct disposal of HLW into repositories is called the ‘once

through cycle’. Nonetheless, most legacy waste produced from nuclear power

plants is in long term storage. This is not a permanent solution, but serves the quest

of buying time to deal with nuclear waste materials some decades in the future [6].

Thus, for a sustainable nuclear power future, reprocessing of spent fuel material is

vital.


Figure 2.1 - The nuclear fuel cycle.

2.2 Nuclear reactors

Nuclear reactors are either thermal or fast reactors. They are classified by their

purpose, by the type of moderator used to slow down neutrons, by the type of fuel

or type of coolant used in them. The main purpose for using nuclear reactors is

power generation; however, they are also used for research, testing and for the

production of materials such as radioisotopes [7]. The principle types of nuclear

reactors and their total net electrical capacities are summarised in Table 2.1.


Table 2.1 - World operational nuclear reactors in 2015 [8].

Reactor

type

Description

Number

of

reactors

Total net

electrical

capacity (MW)

BWR Boiling light-water-cooled and moderated reactor 78 74686

FBR Fast breeder reactor 2 580

GCR Gas-cooled, graphite-moderated reactor 15 8175

LWGR Light-water-cooled, graphite-moderated reactor 15 10219

PHWR Pressurised heavy-water-cooled and moderated reactor 49 24549

PWR Pressurised light-water-cooled and moderated reactor 278 259777

Total 437 377986

The first generation of reactors that was used in the United Kingdom was the gas-

cooled Magnox reactor. The following generation was the GCRs. The first PWR

came much later, at Sizewell B. The majority of nuclear reactors in operation

worldwide are light water reactors, especially PWRs.

Due to the phenomenon of nuclear decay, spent reactor fuel continues to give off

heat even after the reactor has been shut down and the fission reactions have

stopped. The graph in Figure 2.2, below, shows the heat given off by spent fuel

from different reactors after shut down. Naturally, the heat release is highest when

the spent fuel is first removed from the reactor and as time passes, it reduces

significantly.

Figure 2.2 - Heat release from spent nuclear fuel [9].


In order to reduce the decay heat, spent nuclear fuel is stored in cooling water

ponds for a relatively short time when first removed from the reactor. Carbon

dioxide ponds have also been employed; however, they are not always suitable; for

example, they cannot be used for spent fuel from Magnox reactors, as the

magnesium moderator would form magnesium hydroxide [6]. PWRs’ spent fuel

can be stored for a very long time in cooling water ponds, which adds to the

advantages of using them in power plants. Cooling ponds are maintained cool by

using commercial heat exchangers and are easy to operate. However, there is a risk

of reaching ‘criticality’; this is when the pond itself starts acting as a nuclear

reactor, when PWR, BWR and GCR spent fuel is being stored. For reactors that

use natural uranium fuel, such as Magnox and PHWR, there is no criticality

problem [9].

After the cooling off period is over, the fuel is transported from the ponds in safety

flasks to reprocessing and discharge facilities.

2.3 Nuclear reprocessing

Reprocessing of irradiated nuclear fuel first started in the USA, in the 1940s;

precipitation followed solvent extraction was used then. This was to separate

weapons-grade plutonium from spent fuel materials. For the Manhattan Project, the

plutonium-uranium extraction (PUREX) process was employed. In the PUREX

process, uranium and plutonium are extracted, leaving fission products in the HLW

disposal stream [10]. Two decades later, the UK and France adopted and modified

the PUREX process, developing second generation reprocessing plants. Similarly,

at different scales, improvements to the PUREX process took place in Belgium,

Germany, Japan, Russia, China and India. In the 1980s, the UK and France

developed the process further and established third generation civil reprocessing

plants, where discharge of radioactive waste into the environment was reduced

significantly. These facilities produced a mixture of non-weapons-grade uranium

and plutonium called mixed oxide (MOX) fuel [11]. In the UK, however, these

facilities have been closed recently. Table 2.2 shows the nuclear civil reprocessing

plants around the world, and their capacities in 2014.


Table 2.2 - World commercial reprocessing capacity in 2014 [12].

Location (and design) Reactor, fuel type Capacity (t/year)

UK, Sellafield (THORP) Thermal, oxide 600

UK, Sellafield (Magnox) Thermal, metal 1500

France, La Hague (UP2-800, UP3) Thermal, oxide 1700

Japan, Rokhashamura Thermal, oxide 800

Japan, Tokai Mura Thermal, oxide 40

Russia, Ozersk (Mayak) Thermal, oxide 400

India (PHWR, 4 plants) Thermal, metal 330

In the following sections, the two main nuclear reprocessing techniques, aqueous

reprocessing and pyroprocessing are described.

2.3.1 Aqueous reprocessing

In aqueous reprocessing solvent extraction technologies are used. In most spent

fuel reprocessing plants, the fuel is dissolved in nitric acid to aid the process.

Equations 2.1-4 are the main dissolution reactions that take place between spent

uranium species and nitric acid [11]. In the PUREX process, a solvent consisting of

tri-butyl phosphate (TBP) dissolved in hydrocarbon diluent is contacted with a

solution of dissolved nuclear fuel in nitric acid to extract uranium and plutonium,

leaving fission products for disposal [10]. The separation process consists of four

main steps; extraction, scrubbing, stripping and washing. This is demonstrated in

Figure 2.3.

3𝑈𝑂2 + 8𝐻𝑁𝑂3 → 3𝑈𝑂2(𝑁𝑂3)2 +𝑁𝑂 + 4𝐻2𝑂 2.1

𝑈𝑂2 + 4𝐻𝑁𝑂3 → 𝑈𝑂2(𝑁𝑂3)2 + 2𝑁𝑂2 + 2𝐻2𝑂 2.2

𝑈3𝑂8 + 7.5𝐻𝑁𝑂3 → 3𝑈𝑂2(𝑁𝑂3)2 +𝑁𝑂2 + 0.35𝑁𝑂 + 3.65𝐻2𝑂 2.3

𝑈𝑂3 + 2𝐻𝑁𝑂3 → 𝑈𝑂2(𝑁𝑂3)2 +𝐻2𝑂 2.4

Mixer-settlers are used for the extraction. They consist of two compartments and

use equilibrium contactors, where the step concentration profile is the aqueous

phase and the solvent is passed from one stage to another. This technology is

implemented in Sellafield, La Hague and Rukhshamura [11].


Figure 2.3 - Solvent extraction for used nuclear fuel (UNF).

The species in spent fuel that contribute the most to long-term radiotoxicity are the

minor actinides, mainly Am, Cm, Np and Pu. Plutonium is recovered in the

PUREX process, and neptunium can also be recovered in advanced PUREX

processes. Other solvent extraction processes have been developed to extract the

other minor actinides from the lanthanides, such as the transuranic extraction

(TRUEX) for the removal of americium and curium, and the selective actinide

extraction (SANEX) processes [13].

Solvent extraction for nuclear aqueous reprocessing suffers from a few drawbacks,

primarily in terms of nuclear proliferation, as it provides a route for the separation

of weapons-grade plutonium as a single species. Another concern, is that it

employs solvents that contain hydrogen and carbon, which are neutron moderators,

creating a criticality accident risk.


2.3.2 Pyroprocessing

Pyrochemical processing or pyroprocessing is a non-aqueous high temperature

electrochemical method for the refinement and separation of irradiated spent

nuclear fuel. It employs molten salts as solvents [14]. It is generally a desired

scheme as it exploits more compact facilities than in solvent extraction, which is

economically favourable for decontamination, it also requires a shorter cooling

period for irradiated fuel, and it is a criticality accident risk-free process. Different

process systems and salts have been studied in pyroprocessing, most of which are

summarised in a Nuclear Energy Agency (NEA) report [15].

There are currently two main molten salt technology processes in existence, both

using chloride salts as electrolytes; one in the USA at the Argonne National

Laboratory (ANL) using LiCl-KCl eutectic, for metallic fuel for the Integral Fast

Reactor (IFR), and one in Russia at the Research Institute for Atomic Reactors

(RIAR) using NaCl-KCl eutectic, for oxide fuel for the Fast Breeder Reactor

(FBR) [16]. Both processes are outlined in Figure 2.4. Another major difference

between the ANL and the RIAR pyroprocessing systems is the removal of

cladding. At RIAR cladding is removed before the pyrochemical refining stage, as

is the practice in solvent extraction. This creates a radioactive waste stream. At

ANL, it has been proposed that the zirconia fuel cladding is removed in-situ in the

reprocessing stage via chlorination [17]. Equation 2.5 shows this dissolution

reaction [18]. On a smaller scale, advances have been made in molten salt nuclear

pyroprocesses in Europe and Japan as well [19-24].

𝑍𝑟𝑂2 + 2𝐶𝑙2 → 𝑍𝑟𝐶𝑙4 + 𝑂2 2.5

Pyroprocessing is a desirable technology due to its inherent safety features, its

ability to recover most fission products, and also because it is proliferation

resistant, as it does not conventionally allow for the separation of weapons-grade

plutonium as a single species, dissimilarly to the aqueous solvent extraction

reprocessing schemes. Pyroprocessing remains in the R&D and pilot plant scales.

Its facilities also require to be sealed from the atmosphere, and thus maintenance is

very important. It also employs much higher temperatures than solvent extraction.


However, there is a clear motivation for implementing Generation IV nuclear

reactors, which require metal as start-up fuel and follow a fully integrated and safe

reactor and reprocessing design arrangements, hence, advances in the technology

are made.

Figure 2.4 - Closed fuel cycles based on pyroprocessing developed at ANL (right

branch) and RIAR (left branch) (Note: dismounting should read disassembling in the

top box of the diagram) [25].


2.4 Molten salts

Molten salts are used extensively in the chemical industry. Their established

applications are in the production of pure metals, such as aluminium, magnesium

and sodium. They also have other important applications that are not fully

exploited, which include molten salts fuel cells and batteries, catalysis, in the glass

industry, and in solar power and energy storage [26].

In this section, a review of the characteristics and applications of molten salts as

electrolytes is given.

2.4.1 Molten salt electrolysis

Molten salt electrolysis can be used for the electro-winning, refining and plating of

refractory metals (Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W). The conversion of metals

from their ores to their finished refractory stage is both energy and capital

intensive. The electrochemical process scheme is a natural solution for these

demands, as it significantly reduces operating costs through increasing the

efficiency of the recovery of metals [27].

For most refractory metals electro-hydrometal-lurgical routes are not feasible for

production, due to one or more of the following reasons [28]:

1. Most of these metals can only be electrodeposited at potentials more

negative than those required for hydrogen evolution, as thermodynamic

data shows.

2. The metals rapidly become protected by oxide films in oxidising

environments. This can be a problem in fused salt systems too.

3. Metallic ions often get transformed to stable oxy-cations in aqueous

solutions. Metallic ions with low oxidation states reduce water (e.g. Ti2+

),

and those with high oxidation states oxidise water (e.g. Ti5+

).

As a result of these factors, the use of molten salts as solvents for electrolytic

processes in metals production has emerged. The industrial applications of molten

salts have been well acknowledged for more than a century. The commercial

production of Al, Mg, Na, K, Li and Be make use of molten salt electrolysis

processes [29]. However, the major processing routes for most refractory metals


are thermal, although most of them have also been extracted, refined or formed via

molten salt electrolysis in laboratory or pilot plant scale [28].

2.4.2 Advantages of using molten salts as electrolytes

Molten salts are non-aqueous electrolytes. One of their best features is their large

electrochemical window, which allows for a large variety of electrochemical

processes to be carried out, as alkaline earth halides in salts have very negative

Gibbs energies of formation, hence molten salts have a high decomposition

potential, nonetheless this potential can be reduced by lowering the activity of the

metal [28]. There is also a large number of different salts, salt mixtures and salt

eutectics to choose from, with a wide temperature range (e.g. LiCl-KCl molten at

352 ˚C and operational at much higher temperatures). This allows for the various

electrochemical reactions and their rates to be controlled via temperature. Hence,

molten salts are defined into two categories; high temperature salts (above 700 ˚C)

and low temperature salts (below 700 ˚C) [30]. In addition, molten salts have a

good ionic conductivity range (2-9 (Ω cm)-1

) [31].

When comparing molten salts to aqueous solutions, molten salts have the capacity

to dissolve materials to a very high concentration. This high solubility results in a

high limiting current density, and hence high productivity [32]. Chemical reactions

between the metal ions and the solvent are generally absent in molten salt

processes. However, due to the variety of metal oxidation states, interactions

between the metal ions and the solvent must be carefully monitored.

In a reaction vessel, when a cold metal electrode is immersed into the molten salt

solvent, a thin film of solid salt forms on the metal surface, acting as a temporary

insulator, this reduces any thermal shock from taking place. This thin film and the

high heat and wetting properties of salts cause the so called ‘preheating effect’,

where rapid heat transfer through conduction takes place. Similarly, when the

metal electrodes are removed from the vessel a thin liquid film forms on the

surface. This helps in the protection and the cooling down of the final product.

Also, when the reaction vessel is sealed from the environment, oxide formation and

scaling effects of the materials are eliminated. This lessens maintenance problems

[33].


Molten salt processes are also relatively low in initial costs. The reactor vessels

normally have easy accessibility, regardless of how complex the system may be,

and the final retrieved product is typically of high purity [29].

The intensive research on the subject over the past century has determined the

various physical, chemical and thermodynamic properties of molten salt systems to

a high standard. Nevertheless, until recently, not many innovative processes of

using molten salt electrolysis for metal production have been developed [34].

2.4.3 Disadvantages of using molten salts as electrolytes

Despite the many advantages of using molten salts as electrolytes, there are a

number of disadvantages. One of which is that the metal deposits produced by

molten salt electrolysis are usually dendritic and/or powdery mixed with salt,

which leads to a recovery process that is usually energy intensive and requires a

consolidation process, which involves leaching, grinding and floatation procedures.

Large scale molten salt processes can also prove to be difficult, as peripheral

handling facilities are normally employed. Also, the insolubility of the high

valence metal halides (mainly chlorides) can form cluster-type compounds within

the electrolytic melt, which can complicate the process [28]. Another issue is the

formation of CO2/CO at the ‘inert’ anode, which is ordinarily a form of carbon.

This could back-react with the metal deposited.

Due to the corrosive nature of molten salts when exposed to the environment

(oxygen), reactor vessels and electrode materials can become damaged if not

handled appropriately [29]. Serious health hazards can also arise from molten salt

processes. This is due to their high operating temperatures and chemical reactions,

which can yield toxic fumes. However, these hazards can be eliminated by proper

design and planning, taking risk precautions, and the periodic cleaning and

replenishment of salts and equipment [33].

2.4.4 Operating temperatures of molten salts

Frequently, mixtures of molten salts are used instead of a single salt. These

mixtures are binary, ternary and sometimes even quaternary. They usually have

much lower melting temperatures than pure salt constituents. This allows for


manipulations of the system’s chemistry and temperature, to achieve desired

operating designs. Table 2.3 shows the melting points of a few selected salts and

salt mixtures.

Table 2.3 - Melting temperatures of salt mixtures and pure salt constituents [31, 35].

Constituents

Amounts (mol%)

Melting

point (˚C)

Usual

temperature

used at (˚C)

Pure

constituents

Melting

point (˚C)

LiCl-KCl 59-41 (eutectic) 352 450-500 CaCl 772

NaCl-KCl 50-50 658 725-750 LiCl 610

MgCl2-NaCl-KCl 50-30-20 396 475 NaCl 801

AlCl3-NaF 50-50 154 175 MgCl2 714

BF3-NaF 50-50 408 KCl 770

AlF3-NaF 25-75 1009 1080 AlCl3 192*

NaF 995

AlF3 1272*

*Under pressure

Some molten salt mixtures form eutectics, which have a unique minimum melting

point, at a distinctive molar composition of the salt components. LiCl-KCl is such a

eutectic; its phase diagram is depicted in Figure 2.5. Eutectics are common in

chemistry, when two or more species, with their own bulk lattice arrangements,

strike a unique composition forming a superlattice, that can release all its

components at once into a liquid mixture at a much lower distinct temperature [36].

Figure 2.5 - Liquid state domains (non-shaded zones) of salt mixtures [35].


2.4.5 Conductance of molten salts

Molten salts have high ionic conductance, especially chlorides, which makes them

ideal mediums for electrolytic processes. Figure 2.6 shows the specific ionic

conductance of some pure molten salts at different temperatures. The high

conductance of lithium chloride is particularly notable. When using salt mixtures,

their specific conductance can be calculated using Equation 2.6, where a, b and c

are parameters tabulated by Van Artsdaled and Yaffe [37] for different molar

compositions of various salt mixtures and temperature ranges.

𝑘 = 𝑎 + 𝑏𝑇 + 𝑐𝑇2 2.6

Where, k is the specific conductance in mho cm-1

;

T is the temperature in ˚C.

Figure 2.6 - Specific conductance of pure molten salts vs. temperature [37].

2.4.6 Density of molten salts

Molten salts possess relatively low densities, which contribute to them having good

diffusion and dissolution properties. The densities of some molten salt eutectic


mixtures at different temperatures are presented in Figure 2.7. The relatively low

density of LiCl-KCl eutectic is noteworthy, as it inhances transport phenomena in

the fused salt. The density of pure salts and various salt mixtures can be calculated

using Equation 2.7, where a and b are constants also published by Van Artsdaled

and Yaffe [37] for different molar compositions of various salt mixtures and

temperature ranges.

𝜌 = 𝑎 − 𝑏𝑇 2.7

Where, is the density in g cm-3

;

T is the temperature in ˚C.

Figure 2.7 - Density of molten salt eutectic mixtures vs. temperature [38].

2.5 Electrochemical reduction of metal oxides in molten salts

2.5.1 The Fray Farthing Chen (FFC) Cambridge process

The FFC Cambridge process [39] is a novel approach for producing titanium metal

from titanium oxide using molten salt electrolysis. Titanium has many advantages

over other metals, in terms of weight, density, corrosion, maintenance and lifetime

costs. However, its usage remains restricted due to its high production costs, and

hence raw material cost [40]. The conventional method for producing metal


titanium is via the Kroll process [41]. In this process, TiCl4 is reduced with pure

magnesium in a molybdenum-lined crucible, in the presence of pure argon, at a

temperature of 1000 ˚C. It is then separated from magnesium salts by leaching and

acid treatment. The titanium metal in powder form is then compressed into bars

and melted in special vacuum apparatus. The Kroll method for producing titanium

metal is an expensive and complex procedure; Kroll himself predicted that his

process would be replaced by an electrochemical process.

Since the 1950s and for forty years, research was underway in search of a new and

cheaper method for titanium metal production, until the FFC Cambridge process

was discovered in the late 1990s. The FFC Cambridge process is a novel

electrolytic method for reducing metal oxide to metal in a molten salt medium. It

was patented globally in 1998 [42, 43].

For the direct reduction of titanium oxide to metallic titanium, a qualified reducing

agent, R, needs to be selected to remove the oxygen according to Equation 2.8.

𝑇𝑖𝑂𝑥 + 𝑥𝑅 ↔ 𝑇𝑖 + 𝑥𝑅𝑂 2.8

The thermodynamic affinity of oxygen to R should be higher than to titanium.

Reducing agents such as aluminium and carbon pollute and decrease the quality of

the titanium metal produced. Only calcium and rare earth metals can reduce the

remaining oxygen in the metal to less than 1000 mass ppm [44]. The reduction of

titanium oxide chemically using calcium was proposed long before the FFC

Cambridge process, however it was not considered due to the fact that calcium

forms a CaO film on the titanium surface, which physically hinders the successive

reduction by calcium. This is illustrated in Figure 2.8(a). The use of molten CaCl2

salt as the electrolytic medium in the FFC process eliminates this problem, as

CaCl2 can dissolve amounts as large as 20 mol% of CaO and a few mol% of

calcium. Hence, the CaO film would be removed as illustrated in Figure 2.8(b)

[45]. This is the basis of the Ono-Suzuki (OS) process [46].


Figure 2.8 - Calcium reduction of TiO2. (a) Calcium reduction. (b) Calcium reduction

in molten CaCl2.

There are two proposed mechanisms for the electrochemical reduction of titanium

oxide in molten CaCl2. These are as follows:

1. Deposition of calcium at a more cathodic potential followed by a chemical

reaction:

𝐶𝑎2+ + 2𝑒− ↔ 𝐶𝑎 2.9

𝑇𝑖𝑂𝑥 + 𝑥𝐶𝑎 ↔ 𝑇𝑖 + 𝑥𝐶𝑎𝑂 2.10

2. Electrochemical reduction of Ti to release its oxide:

𝑇𝑖𝑂𝑥 + 2𝑥𝑒− ↔ 𝑇𝑖 + 𝑥𝑂2− 2.11

In the first mechanism, the OS process, as presented in Equations 2.9 and 2.10,

calcium is deposited on the titanium oxide cathode; it reacts with the titanium

oxide on the surface forming CaO, which is soluble in CaCl2. In the second

mechanism, the FFC Cambridge process, as presented in Equation 2.11, direct

reduction of titanium oxide is achieved electrochemically. Hence, the chemical

reaction with calcium does not take place [39].

In the original FFC experiment, an electrochemical cell was constructed using

titanium foil as the cathode and graphite as the anode, both were immersed in

molten calcium chloride at 800 ˚C. Cyclic voltammograms were conducted, as

illustrated in Figure 2.9. As can be seen, in the ‘as received’ titanium foil, Figure


2.9(b), cathodic peaks 1 and 2 and their anodic counterpart 7 are absent. These

peaks are predominant in the cyclic voltammogram of the oxidised titanium foil,

Figure 2.9(a). Peaks 1 and 2 appear at a less negative potential than that of calcium

deposition, peaks 4 and 4’ in (a) and (b) respectively. Thus, they are indicative of a

direct electrochemical reduction of TiO2. Peak 7 appears at a less positive potential

than that of Cl2 evolution, peaks 8 and 8’, indicative of a reoxidation process. The

electrochemical cell set-up, Figure 2.10, was later improved using TiO2 pellets as

the cathode and the temperature was raised to 950 ˚C to enhance kinetics.

Figure 2.9 - Cyclic voltammograms of titanium foils in molten CaCl2, scan rate: 10

mV s-1

, at 800 ˚C. (a) Oxide-scale-coated titanium foil. (b) As received titanium foil

[39].

Figure 2.10 - Electrolytic cells for the reduction of TiO2 pellets via the FFC

Cambridge Process [39].


Schwandt and Fray [47] conducted an investigation to determine the kinetics of the

electrochemical reduction of TiO2 to Ti metal in molten CaCl2. Partially reduced

pellet samples were extracted by terminating the reduction process after specified

reaction times. These samples were analysed using X-ray diffraction (XRD),

scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy

(EDS). The experiment was split into three phases where chronoamperommety was

performed. In the first phase, a potential of -2.5 V was applied for 8 h; in the

second phase, the voltage was increased to -2.7 V and applied for 24 h; and in the

third phase it was further increased to -2.9 V and left unchanged for 24 h. It was

deduced that the reactions presented in Equations 2.12-14 took place in the first

phase, which are attributed to a chemical reduction through calcium deposition. In

the second phase, the chemical reaction illustrated in Equation 2.15 took place

concurrently with the electrochemical reaction in Equation 2.16 at a lesser

potential. The third phase involved the decomposition of CaTi2O4 and the

formation of TiO, Equation 2.17. The TiO then further reduced electrochemically

to produce titanium metal with solid solutions of oxygen in titanium, Ti[O]δ, where

δ → 0. Thus, concluding that the reduction of TiO2 to Ti in CaCl2 is a combination

of both, chemical and electrochemical processes.

4𝑇𝑖𝑂2 + 𝐶𝑎2+ + 2𝑒− ↔ 𝑇𝑖3𝑂5 + 𝐶𝑎𝑇𝑖𝑂3 2.12

3𝑇𝑖𝑂2 + 𝐶𝑎2+ + 2𝑒− ↔ 𝑇𝑖2𝑂3 + 𝐶𝑎𝑇𝑖𝑂3 2.13

2𝑇𝑖𝑂2 + 𝐶𝑎2+ + 2𝑒− ↔ 𝑇𝑖𝑂 + 𝐶𝑎𝑇𝑖𝑂3 2.14

𝐶𝑎𝑇𝑖𝑂3 + 𝑇𝑖𝑂 ↔ 𝐶𝑎𝑇𝑖2𝑂4 2.15

2𝐶𝑎𝑇𝑖𝑂3 + 2𝑒− ↔ 𝐶𝑎𝑇𝑖2𝑂4 + 𝐶𝑎

2+ + 2𝑂2− 2.16

𝐶𝑎𝑇𝑖2𝑂4 + 2𝑒− ↔ 2𝑇𝑖𝑂 + 𝐶𝑎2+ + 2𝑂2− 2.17

𝑇𝑖𝑂 + 2(1 − 𝛿)𝑒− ↔ 𝑇𝑖[𝑂]𝛿 + (1 − 𝛿)𝑂2− 2.18

Studies by Dring et al. [48, 49] on thermally grown titanium oxide thin films,

showed that four cathodic peaks associated with the electrochemical reduction of

TiO2 were observed. The cyclic voltammogram is shown in Figure 2.11. C0 is


associated with the decomposition potential of the salt and the deposition of Ca. C4

to C1 are attributed to the corresponding reactions in Equation 2.19. These were

also characterised using XRD, SEM and EDS.

𝑇𝑖𝑂2𝐶4→ 𝑇𝑖3𝑂5

𝐶3→ 𝑇𝑖2𝑂3

𝐶2→ 𝑇𝑖𝑂

𝐶1→ 𝑇𝑖[𝑂]𝛿 2.19

In the initial stages, the formation of CaTiO3 through the chemical reaction of TiO2

with Ca2+

and O2-

was observed. However, the titanate phase was not observed in

the later stages with TiO. It was established that the diffusion of oxide ions in the

product titanium-oxygen was the rate determining step. The formation of CaTiO3

was due to the high concentration of oxide ions on the surface, and displaced the

reduction reaction of Ti2O3 to TiO with reactions that possess significantly more

negative potentials than predicted by the bulk oxide content of the salt. Under such

high local oxide activity, the calciothermic reduction of titanium oxide, due to the

formation of elemental calcium, was substantial. Thus, the reduction process was

also determined to be a mixture of both, chemical and direct electrochemical.

Figure 2.11 - Cyclic voltammograms of TiO2 and inert electrode in CaCl2, scan rate:

50 mV s-1

, at 900 ˚C [49].


An in-situ synchrotron diffraction study was conducted by Bhagat et al. [50],

which shed more light onto the reduction pathway of TiO2 to Ti in CaCl2. The

precursors in the study were in the form of pellets. The most important finding

from this study was that it confirmed the presence of the CaTiO3 titanate species,

from the early stages of the process until almost the very end, before the final

reduction of TiO, which was different from findings in previous works.

Stoichiometric analyses of the results also showed that the formation of CaTi2O4

was due to the reaction of CaTiO3 with TiO, rather than to a direct electrochemical

reduction of the titanate species.

Many studies on the FFC Cambridge process were undertaken to develop the

understanding of it and its different uses. Studies on the extraction of titanium from

different pellet and sponge-like precursors [51-53] were carried out. Also,

investigations using cheap precursors, such as titania dust and titanium-rich slug

[54]. It was noted that some metallic impurities, such as aluminium and

manganese, were partly or completely removed during the electrolysis. In addition,

the FFC Cambridge process was successfully employed, using mixed oxide

precursors, to produce NiTi [55-57], Ti-Mo alloys [58] and Ti-W alloys [59].

Figure 2.12 - Stages of the FFC Cambridge process [60].


The patent [43] on the FFC Cambridge process claims that a number of metals and

semi-metals, such as Ti, Si, Ge, Zr, Hf, Sm, U, Al, Nd, Mo, Cr and Nd, can be

produced with the starting material being the metal oxide, nitride, sulphide or

carbide in a fused salt system. However, the majority of the research that has been

reported so far has been on metal oxides, especially titanium dioxide. The FFC

process for the production of titanium metal has been successfully scaled up, and is

being industrialised by multiple developers. Figure 2.12 illustrates the simple

stages that occur in the FFC Cambridge process for titanium metal production.

2.5.2 The three-phase interline (3PI)

The initial three-phase interline (3PI) model was proposed by Chen et al. [61]. It

was developed by Deng et al. [62], and a comprehensive model by Kar and Evans

[63] then followed. Any two phases are connected by a two-dimensional plane, and

any three phases can only be connected by a one-dimensional point or line, the 3PI.

The 3PI connects a conducting solid metal phase (SMP), an insulating solid

compound phase (SCP) and a liquid electrolyte phase (LEP). In the case of a

molten salt system, these are the metal phase (current collector), the metal oxide

phase and the fused salt phase. This is illustrated in Figure 2.13.

Figure 2.13 - Schematic representation of a three-phase interline (3PI) connecting a

solid metal phase, a solid compound phase (metal oxide), and a liquid electrolyte

phase (molten salt). The grey planes are the interlines between two neighbouring

phases. At the 3PI, electron transfer occurs between the metal and the metal oxide and

oxygen anion transfer between the metal oxide and the molten salt.


Along the 3PI, electron transfer occurs between the metal and the metal oxide, and

oxide anion transfer occurs between the metal oxide and the molten salt. The 3PI is

vital for the electrochemical reduction reaction. The longer the 3PI, the larger the

current flow. As a reaction proceeds, the length and shape of the 3PI changes. The

3PI model, however, does not account for any inherent electrical conductivity of

the oxide species. It also overlooks any changes in electrode morphology that takes

place. Nonetheless, the model agrees with experimental findings to a reasonable

level.

2.5.3 Challenges for electrochemical reduction in molten salts

There are a number of issues that arise when using a standard reactor setup,

whether using electrode rods, porous pellets or sponge-type precursors as

electrodes. In the case of reducing titanium oxide via the FFC Cambridge process,

the current efficiency is quite low, 10-40%, to achieve sufficiently low oxygen

content, 0.3%, in the final titanium product [51]. The current efficiency and the

speed of the process must be increased in order to allow the electrochemical

reduction of refractory metals in molten salts to be applied industrially [64]. This

low efficiency and low speed of the process could be due to a number of reasons,

such as diffusion, oxygen ionisation, electrolysis of molten salts, metal-to-oxide

molar volume ratio, and transport characteristics of such systems. However, the

most convincing explanations, from experimental results obtained from works on

this matter are: the limited diffusion of the electrolyte within the precursor porous

matrix, thus leaving the inner parts unreduced [65, 66]; and the anomalous electro-

migration of oxygen vacancies in reduced metal oxides [67-69].

As a reduction reaction proceeds, the 3PI propagates, this is demonstrated in Figure

2.14, outward from the current collector to the inner parts of the precursor, and

then inward from the reduced metallic surface towards the precursor’s centre. The

current increases initially, plateaus, then reduces until it stops [61]. This is due to

the electrolyte not diffusing to the inner parts of the electrode, hence, the absence

of the 3PI. This is due to the nature of the morphology of the oxide precursor, and

to the change in density and structure of the materials reduced on the surface. For

example, some metals, tend to have much higher densities than their oxides,


causing the metallic surface to collapse onto itself, creating a much denser

morphology, hindering the diffusion of electrolyte [56].

Figure 2.14 - Schematic of the 3PI propagation mechanism of a cylindrical metal oxide

pellet. (a) Propagating from the current collector along the surface. (b) Propagating

within the pellet [61].

Precursors in the form of oxide films, pellets or sponge-types are all porous. The

size and interconnectivity of these pores affects the reduction process [51, 52].

When the metal oxide is reduced, some of the oxide ions transfer into the melt and

migrate to the anode; however, some of them are trapped in the cathode’s pores, as

was observed by Dring et al. [49], where it was noted that the concentration of

oxide ions at the surface of the electrode was higher than in the bulk molten salt.

This can change the potential needed for the reduction significantly (refer to

predominance diagrams, Chapter 4). It can also cause the formation of other metal

phases (e.g. calcium titanate) due to the salt cation reacting with the oxygen ions on

the surface. Ultimately, the oxide ions on the surface could block the current

transfer, bringing the reduction process to a halt and leaving the inner parts of the

metal oxide precursor unreduced.

Thus, creative new process designs must be developed to improve the efficiency

and performance of the electrochemical reduction of metal oxides. One solution

could be the use of a ‘fluidised cathode’ process [70], which was developed within

the scope of this thesis.


2.6 ‘Fluidised bed’ electrochemical processes

Numerous experimental, theoretical and review studies have been published on

fluidised bed electrochemical processes [71-79]. The main advantages of

employing fluidised bed electrodes are their large specific area (high specific

productivity), and the free flowing character of their fluidised bed structure [80].

Applications include fuel cells, hydrogen peroxide synthesis [81, 82], water

purification and organic electrosynthesis [83, 84]. In fuel cells, the fluidised

particles are sometimes coated with catalyst to enhance the process [74]; their main

uses are as fluidised bed anodes for carbonate fuel cells [85-88] and cathodes for

alkaline fuel cells [73, 74, 89, 90]. In water purification a large tank containing a

fluidised bed of particles is commonly used. These particles are charged

cathodically by a feeder electrode, as the waste water flows through the tank, metal

ions are absorbed on the surface of the charged particles. When these particles then

come into contact with the working electrode, the potential drives a charge transfer

reaction and the unwanted metals are deposited as discharge. The particles need

replacing with fresh ones very often in such continuous processes [91].

Fluidised bed electrochemistry has been extensively studied and used in the

recovery of copper and other metals from dilute solutions for environmental

applications [92]. The metals to be eliminated are usually at low concentrations

(less than 1 g l-1

) [80]. A schematic of the process is demonstrated in Figure 2.15.

The metal recovery process operates in a similar way to fluidised bed electrodes in

water purification technology, where the metal ions are attracted to the charged

fluidised bed particles and form a layer on the outside of the particle. There is a

continuous feed and recovery of the metal in dilute solutions by the introduction of

small particles at the top of the bed and the removal of grown particles at the

bottom of it [93, 94]. In this thesis, the particles themselves are reacted and reduced

from the metal oxide to the metal phase.

Some disadvantages in using fluidised beds in pyroprocessing could be the

difficulty of separating the materials from the salts, and the risk of creating small

free flowing particulates of fission products that could escape, or get carried by

gases.


Figure 2.15 - Fluidised bed electrolysis reactor according to Fleischmann and

Goodridge [95].

Fluidised bed electrodes have been studied and used industrially in various

applications, though their definition is not precise; for example, sometimes they are

reducing agents and other times they act as catalysts, yet they are still loosely

called ‘fluidised bed electrodes’. They have not been applied in molten salt

electrochemical reduction processes, yet they may be a viable route to increasing

the performance of such processes.

In the proposed arrangement, metal oxide particles are suspended in the molten salt

via the use of an agitator (e.g. inert gas bubbling, stirring). An inert current

collector, held at a suitable potential, is used, and an anode separated in its own

compartment, to stop the reoxidation of the reduced metal particles. A schematic of

the fluidised cathode process, and the different paths that a metal oxide particle

could follow to be reduced, is illustrated in Figure 2.16. When a metal oxide

particle is suspended and agitated in the fused salt, it comes into contact with the

current collector, where a 3PI is instantly initiated, it is then either reduced fully or


partially, and it could get deposited on the electrode’s surface or reflected from it

back into the bulk salt. This process can be repeated multiples times for all

particles until the reduction reaction reaches completion. The final product can then

be retrieved from two areas: the deposit on the current collector’s surface and the

bottom of the reactor crucible, as particles sink and settle due to the high density of

refractory metals.

Figure 2.16 - Schematic of a fluidised cathode process in a molten salt showing various

reaction mechanisms between particles in the melt and the electrode.

Studies on the electrochemical reduction of metal oxide particles on surfaces have

recently been published [96-98]. However, these particles were small in size, not

fluidised, and were used for thin film preparation. The fluidised cathode process is

a larger-scale three-phase metal oxide to metal reduction process. A 3PI is

continuously being created, every time an impending metal oxide particle comes

into contact with the current collector. The matrix of the fluidised cathode also

elevates the diffusion of electrolyte within the metal oxide particles, and of the

oxygen ions within the electrolyte. Another proposed advantage of using a

fluidised cathode process is that it would eliminate certain steps, in the preparation

of the precursor and recovery of the reduced metal in the FFC Cambridge process,

such as steps 3, 4 and 5 in Figure 2.12, with associated cost reductions.


2.7 Spent nuclear materials in molten salts

A significant amount of research on fission products in fused salts has been carried

out over the past century. Molten salts provide a stable, proliferation resistant

electrolytic medium for the dissolution, electro-plating, electro-reduction and

refining of spent nuclear materials.

In an early study by Inman et al. [99], known quantities of UCl3 were dissolved in

an alkaline earth metal chloride eutectic, and a pure uranium working electrode

was used. Following Equation 2.20, uranium was deposited on the working

electrode. At current densities less than 100 mA cm-2

, the process had 100%

Faradaic efficiency, based on the total deposit (adherent and non-adherent on the

electrode surface) and the Nernst equation for the 3-electron-transfer determining

step in Equation 2.20.

𝑈 ↔ 𝑈3+ + 3𝑒− 2.20

The cell potential at 452 °C was determined to be -1.398 V (vs. 1 wt% Ag/Ag+

reference electrode). It was also observed that some of the product was formed

from a reaction with lithium in the salt, which resulted in a powdery deposit, and

some of the product formed at lower potentials directly at the electrode resulted in

a dendritic deposit. A similar, more recent study by Kuznetov et al. [100], using a

tungsten working electrode, determined that the reduction potential for Equation

2.20, in a LiCl-KCl eutectic at 500 °C was ~ -1.5 V.

Most research on spent nuclear materials in molten salts was developed by the

ANL [101-103]. Their molten salt electrorefiner, the schematic of which is

presented in Figure 2.17, was a great innovation in nuclear molten salt technology.

The electrorefiner comprises two cathodes: a solid steel/iron cathode, and a liquid

cadmium cathode. Spent nuclear fuel is chopped and placed in a basket at the

anode. This is then anodically oxidised. The actinides are reduced at the bottom of

the electrorefiner at the liquid cadmium pool. Uranium is then electro-transported

to the steel cathode, and a mixture of U, Pu and other actinides are transported to

the liquid cadmium basket, and the Cd pool acts as the anode. Less noble fission

products (e.g. alkali metals and alkaline earth metals) remain oxidised in the salt;


while the metals in the fuel cladding alloy and the noble fission products (e.g.

ruthenium and palladium) are not oxidised and remain in the anode basket, or sink

to the bottom of the electrorefiner as sludge in the cadmium pool. The deposit of

uranium on the steel cathode was dendritic and sufficient for retrieval; however,

once the process was applied to mixed fission products, especially plutonium, the

morphology of the deposit was not adherent; therefore, the liquid cadmium cathode

was introduced.

Figure 2.17 – Schematic of the electrorefining process developed by the ANL,

operated at 500 °C, where AM = alkali metals, AEM = alkaline earth metals, MA =

Np, Am and Cm, NM = noble metals, RE = rare earth metals, FP = fission product

[104].

The electrorefiner was studied and developed further by Koyama et al. [19, 20,

104], they replaced the steel electrode with a pure iron cathode; and by other

researchers [105, 106], who established the potentials at which the actinides and

the cladding materials (e.g. Zr) are reduced from their chlorides, after they are

anodically oxidised. These redox potentials are summarised in Table 2.4. One

important thing to take notice of is that the potentials for the same redox reactions

differ slightly from one study to another (e.g. for the U (III) / U(0) couple, from


Inman et al., Kuznetov et al. and Koyama et al.). This is due to the fact that cell

potentials in molten salt systems are strongly dependant on the O2-

ion activities in

the fused salts, which are difficult to control and are sometimes not accounted for

after salt treatments have been applied. This phenomenon is discussed thoroughly

in thermodynamically produced predominance diagrams, in Chapter 4.

Table 2.4 – Redox potentials of relevant elements in LiCl-KCl eutectic salt at indicated

operating temperatures (V vs. Ag/Ag+ reference electrode) [104-107].

400 °C 450 °C 500 °C

U (III) / U (0) -1.274 -1.233 -1.190

Pu (III) / Pu (0) -1.591 -1.543 -1.497

Np (III) / Np (0) -1.472 -1.434 -1.390

Am (II) / Am (0) -1.592

Zr (II) / Zr (0) -0.693

There are six types of Generation IV nuclear reactors and power plant under

development, these are: the very-high-temperature reactor (VHTR), the sodium-

cooled fast reactor (SFR), the supercritical-water-cooled reactor (SCWR), the gas-

cooled fast reactor (GFR), the lead-cooled fast reactor (LFR), and the molten salt

reactor (MSR). These designs and their operating conditions are summarised in

Table 2.5. The fuel type used in the majority of these reactors is metal. Considering

the fact that most reactors currently in use utilise metal oxide (MOX) fuel and most

of the nuclear legacy waste is from MOX fuel, the conversion of metal oxides to

metals is an important parameter for the development of safer and more efficient

nuclear power generation. As discussed in Section 2.3, pyroprocessing of spent

nuclear fuel provides an inherently safe technology that is also resistant to nuclear

proliferation. Thus, the reduction of spent fuel metal oxides to metals is of

importance to the development of civilian nuclear technology.


Table 2.5 – Generation IV reactor designs under development [108].

Reactor Neutron

spectrum

Coolant Temperature

°C

Fuel cycle Fuel type Size

(MWe)

VHTR Thermal Helium 900 – 1000 Open Oxide 100 – 300

SFR Fast Sodium 550 Closed Metal/oxide 50 - 1500

SCWR Thermal/fast Water 510 – 625 Open/closed Oxide 1000 – 1600

GFR Fast Helium 850 Closed Metal 1000

LFR Fast Lead 480 – 800 Closed Metal 20 – 1200

MSR Fast/thermal Fluoride

salts

700 – 800 Closed Metal* 1000

*The metal is dissolved in the fluoride salt (e.g. UF4 and ThF4).

Numerous studies on the electrochemical reduction of spent fuel oxides in molten

salts have been published. In a study by Hermann et al. [109], crushed spent fuel

was loaded into a stainless steel basket and submerged in molten LiCl-1 wt% Li2O

at 650 °C. A platinum anode and a Ni/NiO reference electrode were used. They

determined that Equation 2.21, for the direct electrochemical reduction of UO2 to U

metal, took place at a potential of -2.40 V, and that the potential required for Li

deposition from the salt, Equation 2.22, was -2.47 V. Thus, there was only a

potential difference of 70 mV between the two reactions, which proved that the

direct electro-reduction was difficult to achieve. However, the chemical reduction

via Equation 2.23 provided supplementary help to the entire process. The only

disadvantage being that the lithium evolution resulted in attacking the platinum

anode and dissolving it. Choi et al. [110] managed to reduce 17 kg of UO2 to U

metal in LiCl-Li2O. They concluded that a small pellet size, with a high anode

surface area resulted in higher current efficiencies.

𝑈𝑂2 ↔ 𝑈 + 𝑂2 2.21

𝐿𝑖2𝑂 ↔ 2𝐿𝑖 +1

2𝑂2 2.22

4𝐿𝑖 + 𝑈𝑂2 ↔ 𝑈 + 2𝐿𝑖2𝑂 2.23

Studies on the reduction of U3O8 were also published. Seo et al. [111] conceived

the main reduction potential for reducing U3O8 to U to be -2.27 V. In a further

study by Jeong et al. [112], 20 kg of U3O8 were reduced successfully to U, with


more than 99% conversion. The reducing potentials seemed to vary from -2.47 V -

-3.46 V. They concluded that an increase in the size of the pellets used in the

cathode inhibits the diffusion of electrolyte to the inside of them, leaving them

unreduced. This can be explained by the 3PI theory, described in Section 2.5.

In a study published by Sakamura et al. [113], UO2 was reduced to U in both CaCl2

and LiCl. The reduction in CaCl2 appeared at < 0.6 V vs. Ca/Ca2+

at 800 °C.

However, the conversion was not successful enough, as the diffusion of oxygen

and electrolyte in the precursors appeared to be problematic. In LiCl, the reduction

potential was < 0.15 V vs. Li/Li+ at 650 °C. The deposition of lithium was

observed, and the process in LiCl had a higher current efficiency and better

electrolyte diffusion in the precursor. Nonetheless, the reduction potentials in both

systems were established to be very close to the salts’ decomposition potentials.

Hur et al. [114] carried out the reduction of UO2 to U metal in molten LiCl-KCl-

Li2O at 520 °C. The reduction potential was at -1.27 V vs. a Li-Pd reference

electrode. They established that the reduction process was entirely caused by the

chemical reduction reaction of UO2 with Li. The contradictions in reducing

potentials in the different studies is again due to the fact that the activity of O2-

ions

in the molten salts is not accounted for, which can change the required potentials

for certain reactions. This is explained in Chapter 4. When the reduction potentials

and the salt’s decomposition potential are close to one another, as is the case for the

reduction of UO2 in fused salt systems, the activity of O2-

ions also affects the

kinetic pathways and rate determining steps of the process. However, the most

important conclusion to draw from this, is the fact that the reduction of UO2 to U

metal, appears to take place in one overall direct 4-electron-transfer step, without

any intermediate uranium oxides being formed (e.g. UO).

2.8 Summary

To set the theme for this research, a background review on nuclear energy and

reprocessing technologies has been provided, molten salts and their applications

have been covered, electrochemical reduction processes for metal productions in

molten salts have also been covered, some fluidised bed electrode technologies


have been included, and finally the electrochemistry of spent nuclear materials in

the literature was also included.

The electrochemical reduction of spent nuclear materials is an important

development for civilian nuclear applications, especially for next generation power

plants. Molten salts provide an excellent electrochemical route for the conversion

of spent fuel to reusable metal fuel, as they are safer, smaller, and produce less

waste streams than aqueous reprocessing routes.

The aim of this thesis is to develop the understanding of spent nuclear materials

behaviour in molten salts, via thermodynamic and electrochemical techniques, and

to investigate electrochemical reduction processes from metal oxides to metals,

which can prove to be significant in future Generation IV nuclear power plants.

New reactor designs are investigated as well, to improve the efficiency of such

processes.

3. Experimental 35

3. Experimental

This chapter is divided into three sections. The main electrochemical techniques

used in experiments are described, the material characterisation techniques used are

also described, and the experimental set-ups and designs are also covered.

3.1 Electrochemical techniques

The majority of the electrochemical studies in this work are conducted using

controlled potential techniques. The chosen potential is set into the potentiostat’s

control, and in turn it supplies the required charge to reach this potential. The

systems studied comprise an oxidised species O and a reduced species R, with

redox reaction O + ne- ↔ R, and associated reduction potential EO/R.

3.1.1 Linear sweep and cyclic voltammetry

Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) are two commonly

used techniques, and are usually the first electrochemical characterisation processes

used to study a system. Both have linear potential versus time functions. In CV, the

potential is reversed and cycled between two set potentials at a constant rate (V s-1

).

The current response occurs at an array of potentials at which the over-potential is

increasing, and in the absence of other charge transfer reactions, this current

reaches a maximum, then starts to decline until it flattens out. Figure 3.1 (a-b)

shows the potential versus time waveforms for LSV and CV for a reversible

(Nernstian) charge transfer reaction.

3. Experimental 36

Figure 3.1 - (a) Potential vs. time waveform for linear sweep voltammetry (LSV). (b)

Potential vs. time waveform for cyclic voltammetry (CV). (c) Current vs. time

response corresponding to LSV. (d) Current vs. potential response corresponding to

CV.

In Nernstian processes, electron transfer is rapid, thus the diffusion of the electro-

active species into the electrolyte is rate determining. In irreversible processes,

electron transfer is very slow, and quasi-reversible processes are both diffusion and

charge transfer limited. In LSV and CV, the diffusion layer at the electrode surface

is not allowed to reach equilibrium, due to the sweep rate of such scans.

For the reversible reaction O + ne- ↔ R, the concentration gradient profile

presented in Figure 3.2 depicts the depletion of O.

The flux of O into the electrolyte is directly proportional to the concentration

gradient of O at any time; this is described by Fick’s first law of diffusion,

Equation 3.1, where the diffusion coefficient of O, DO, is constant.

3. Experimental 37

𝑞(𝑥, 𝑡) = 𝐷𝑂𝜕𝐶𝑂(𝑥,𝑡)

𝜕𝑥 3.1

The flux is measured as current in the external circuit. Under fixed conditions of

potential, the flux would diminish as the concentration gradient decreases due to

diffusive mass transfer of O. In LSV and CV, the potential varies with time, thus

giving O a concentration profile over time at the electrode surface. When the

concentration profile reaches curve 4 in Figure 3.2, the concentration gradient is at

its maximum, which gives rise to the current peaks in Figure 3.1 (c-d).

The main advantage of CV over LSV is that for Nernstian reaction O + ne- → R, it

shows the coupled peak for the reoxidation reaction R → O + ne-, for the redox

couple. It also gives better insight into whether a reaction is in fact

electrochemically reversible, irreversible or quasi-reversible (i.e. only part of the

reverse reactions steps can take place).

As the electron transfer process becomes slower, the cathodic and anodic peaks

broaden and the separation between them becomes larger, due to higher over-

potentials.

Figure 3.2 - Oxidised species (O) concentration vs. distance at different times, 1-5,

during a linear sweep voltammetry.

3. Experimental 38

3.1.2 Chronoamperometry

In chronoamperometry, a set constant potential is applied for a specified time

period, and the current response is measured. For the redox reaction O + ne- ↔ R,

with reduction potential EO/R, at potentials less than the reduction potential, no

reaction would occur; however, at a range of potentials higher than EO/R, the

reduction reaction of O would occur rapidly, with associated charge transfer. The

current versus time response for chronoamperometry is illustrated in Figure 3.3.

The rate at which O moves to the electrode surface is limited by diffusive mass

transfer, which the amount of current flux is proportional to.

The concentration profile of O over time, and the diffusion limited current iD, can

be described by Fick’s second law of diffusion, Equation 3.2, where the boundary

conditions are defined as: lim x → ∞, CO(x,t) = Cbulk and CO(0,t) = 0.

𝜕𝐶𝑂(𝑥,𝑡)

𝜕𝑡= 𝐷𝑂

𝜕2𝐶𝑂(𝑥,𝑡)

𝜕𝑥2 3.2

The advantage of using chronoamperommetry over linear sweep techniques is that

it allows for the electrode to reach polarisation, and it is a more adequate

methodology for analytical characterisation.

Figure 3.3 - Current vs. time response for chronoamperommetry.

3. Experimental 39

3.2 Material characterisation techniques

Following the electroanalytical techniques, material characterisation techniques

were used to identify materials and phases before and after experiments were

conducted. The three techniques used here are: X-ray diffraction for phase

identification; scanning electron microscopy, coupled with energy dispersive x-ray

spectroscopy, for microstructural measurements and phase identification; particle

size analysis for size distributions of powder particles.

3.2.1 X-ray diffraction

Powder X-ray diffraction (XRD) is a very important tool for chemical analyses to

provide evidence for electrochemical processes. The basic principles behind it can

be described using Bragg’s law, Equation 3.3, where n is the order of diffraction

(normally this is unity), λ is the wavelength of the radiation, d is the inter-planar

spacing of the crystal being analysed, and θ is the angle between the diffracted and

incident ray and the crystal plane. This is illustrated in Figure 3.4.

𝑛𝜆 = 2𝑑 sin𝜃 3.3

When n is 1, the scattered beams from successive planes will each travel a distance

differing by one wavelength. The beams will constructively interact registering an

intense diffraction beam at the X-ray detector.

The apparatus used for XRD analysis is a Stoe StadiP capillary geometry system

with a molybdenum source. Eva (Bruker) software, coupled with Mercury

software, and database from the Royal Society of Chemistry (ICSD data base) were

used to analyse and match the diffraction data for phase identification with patterns

from the literature. The sample to be analysed was ground into fine powder and

placed inside a 0.3 or 0.5 mm in diameter capillary, which was then sealed to

protect sensitive samples from air and moisture. The capillary was mounted in a

sample holder and was continuously rotating when measurements were taken.

Typical parameters used were: a measurement angle of 2 – 50 ° with 0.5 ° step, and

20 s step-1

.

3. Experimental 40

Figure 3.4 - X-ray diffraction schematic of a crystal.

3.2.2 Scanning electron microscopy and energy dispersive X-ray spectroscopy

Samples were analysed with scanning electron microscopy (SEM) using a Carl

Zeiss XB1540 apparatus, which is also equipped with a “cross-beam” focused ion

beam (FIB) microscope for microscopic milling of surfaces. Samples in the form of

fine powders were mounted simply by placing them on carbon adhesive discs that

are mounted on the SEM stubs. Working electrodes and current collectors were

mounted in epoxy resin. They were placed in a desiccator that is fitted with a

vacuum pump and left for about 10 minutes to allow the resin to penetrate through

the pores in the samples and to remove air bubbles in the epoxy. After drying, the

sample was ground using silicon carbide grinding paper, starting with a 220 grit,

the sample was ground until smooth, then ground again in a direction perpendicular

to the previous one using a 500 grit. This was repeated using 800, 1200, 2400 and

4000 grits. The samples were inspected under an optical microscope throughout the

grinding procedure. Before placing a sample in the SEM chamber, its surface was

gold coated, using a gold coating machine.

Quantitative analyses were performed using X-ray energy dispersive spectroscopy

(EDS), which was calibrated against a cobalt standard before every use. It was

typically operated at ~20.00 keV and ~15 mm working distance.

3.2.3 Particle size analysis

Samples were analysed using a Beckman Coulter LS13320 laser diffraction particle

size analyser. Laser diffraction correlates the patterns of scattered light and its

3. Experimental 41

intensity at different angles to the particle size distribution of a sample. The

apparatus is fitted with a PC for data collection, which provides particle size

distribution data using the Mie theory (this is explained in the Beckman Coulter

LS13320 manual which can be found online). Small powder samples were

dispersed in deionised water in a special sample holder through which laser is

scattered and detected.

3.3 Experimental design

The experimental set-up was designed to exploit the physical properties of the salt

eutectic, mainly the low melting point of the LiCl-KCl eutectic (352 °C). This

allowed the use of borosilicate glass as the cell wall material, which was beneficial

as it provided visual access to the process.

The rig consisted of three compartments. The crucible: a disposable container

placed inside the cell envelope which contains the molten salt; the envelope: the

outer container of the electrochemical cell, which seals the process from the outer

environment and was used as a safety precaution to contain the molten salt in case

of crucible breakage; the heating element: two different heating methods were

used, one employing a thermostatic salt bath, and one using a vertical tube furnace.

Although the reaction cell was sealed with a constant stream of argon passing

through the top part of it, the experiments utilising the thermostatic salt bath were

carried out inside a dry glove-box, Figure 3.5. This limited water, or any other

types of contaminants, from entering the reaction vessel. The glove-box was made

of Perspex, providing extra shielding when handling uranium, and also protected

against heat given off from the rig.

Figure 3.6 illustrates the general full schematic of the experimental rig. There were

two loops; a closed one where the air inside the glove-box was pumped through

two desiccant tubes in series, to absorb water vapour and reduce humidity, then

was recycled back into the glove-box; and an ‘open loop’ where dry argon was

passed to the inside of the reaction cell, via a flowmeter creating an argon blanket

at the top of the cell, and then out of the reaction cell and the glove-box into a fume

cupboard, whilst passing over two Dreschel bottles to prevent any back flow. There

were two cooling fans at opposite corners of the glove-box to aid the distribution of

3. Experimental 42

temperature, and prevent the Perspex walls from reaching critical heat.

Thermocouples were placed in all sections of the rig to monitor the temperature.

There was also a camera mounted and pointed toward the reaction cell, ready to

record any visual activity.

Figure 3.5 - Perspex dry glove-box containing molten salt electrochemical cell.

3.3.1 Heating and temperature control

Two methods were used for heating the electrochemical cell components to the

desired temperatures. One method uses a thermostatic salt bath, similar to the

arrangement used by Inman and Bockris [115], which benefits from visual

accessibility throughout experiments. The other uses a typical vertical tube furnace,

which is more practical in terms of time and reliability; however, one cannot

inspect the process visually whilst experiments are running.

3. Experimental 43

Figure 3.6 - Schematic of the rig set-up, showing components inside and outside the

dry glove-box.

3.3.1.1 Thermostatic salt bath

The thermostatic salt used was a NaNO3-KNO3 eutectic. It was kept and melted in

a 5 L borosilicate glass heavy-duty beaker, to maintain visibility. Corrosion and

attack effects from the salt on the glass were negligible [116], hence the container

beaker could be used for a large number of experiments. The salt eutectic, once

molten, is homogeneous in temperature and has the ability to store energy for a

long period of time [117]. The eutectic is equimolar and has a melting temperature

of 220 ˚C, as illustrated in the phase diagram in Figure 3.7. It has a latent heat

value of 97 J g-1

. Information on its viscosity and specific heat values at different

temperatures can be found in the works by Coscia et al. [118] and Nissen [119].

3. Experimental 44

Figure 3.7 - Phase diagram for NaNO3-KNO3 eutectic [120].

To heat up the thermostatic salt bath an immersion heater was used. The tubing

material for the immersion heater was silica glass, which withstands higher

temperatures and has higher mechanical integrity when compared to borosilicate

glass. The dimensions of the immersion heater are shown in Figure 3.8. The reactor

cell, when placed in the thermostatic salt bath, is encircled by the bottom round

side of the immersion heater, Figure 3.10.

Figure 3.8 - Dimensions of silica tubing for immersion heater.

3. Experimental 45

Figure 3.9 - Schematic for heating apparatus.

Nichrome wire, 0.46 mm thickness and 4 m long, was coiled up and put through

the silica tubing. This was then connected to a power source and voltage was

passed through it via a variac, as illustrated in Figure 3.9. A small light bulb was

also connected to the top of the immersion heater, to indicate when the power was

on, for safety. Calculations were carried out, Appendix A, to estimate the voltage

needed to be passed through the nichrome wire to reach specific temperatures of

the thermostatic salt bath. This was set at the variac and presented in Table 3.1.

Nonetheless, these are estimates as the heat loss to the environment was not

accounted for. Thus, these calculated values provide an indication of the voltage

needed, and during experimentation the heating is monitored, via the use of

thermocouples, to be able to control the temperatures precisely.

Table 3.1 – Guideline for voltage required to achieve temperatures of 3 kg of NaNO3-

KNO3 thermostatic salt bath after 1.5 h.

T (˚C) 100 200 300 400 500 600

V (V) 44 65 90 102 113 123

Thermocouples were placed in different positions in the rig, as illustrated in Figure

3.6. There were two inside the main rig testing area: one inside the crucible of the

cell, to monitor the operating temperature, and one inside the thermostatic salt bath

beaker. The operating temperature could then be adjusted accurately via the variac.

In practice, the voltages required were much higher than the calculated ones, due to

losses of heat to the environment. There were more thermocouples placed around

the rig, to monitor the process and prevent reaching critical temperatures. These

3. Experimental 46

were placed at the front, back, sides, roof, bottom and corners of the Perspex glove-

box.

Figure 3.10 - Immersion heater rig.

3.3.1.2 Vertical tube furnace

A vertical split-tube furnace was used to provide heat to the electrochemical cell

(CSC 12/90/300 V furnace, Lenton). This was custom built to fit the reaction cell.

It provided a faster heating rate and a stable environment for electrochemical

processes. Measuring the temperature inside the crucible via a thermocouple

showed that the difference between the salt temperature and the set temperature of

the furnace is ~10 °C. This furnace does not afford optical access to the inside of

the cell.

3.3.2 Electrochemical cell

The electrochemical cell was mainly constructed from borosilicate glass. It consists

of the envelope, the crucible, a cell head and electrodes. The crucible was a 250 ml

high-form borosilicate glass beaker (Schott Duran). It was treated as dispensable,

due to breakage caused by salt shrinkage as it solidifies. The crucible is placed

inside the envelope. The envelope was a tall borosilicate glass beaker with a flange

(GPE Scientific Ltd), and was designed to the specifications presented in Figure

3.11.

3. Experimental 47

Machinable ceramic was used for making the cell head (Unfired Pyrophylite,

Ceramic Substrates and Components Ltd). It was supplied in the form of a tube

with 12 cm diameter. Discs of 0.5 cm thickness were sliced off the tube, then holes

were drilled through them with the desired dimensions and arrangement. The cell

head was then baked in a furnace (Vecstar VF1), at a ramp rate of 50 °C h-1

until

400 °C, followed by 100 °C h-1

until 1000 °C, then left at 1000 °C for an hour.

Schematics of the two different cell head arrangements used are illustrated in

Figure 3.14. Silicone rubber suba-seals (Sigma-Aldrich) were used to seal the holes

in the ceramic head, and the required electrodes and thermocouple were pushed

through them. They can withstand temperatures of up to 200 °C continuously or

400 °C intermittently, and it is easy to move the electrodes up and down through

them. A PTFE gasket (Sci-Labware) was placed between the envelope flange and

the cell head, which provided sealing from the environment. They were held

together using a metallic clamp (Sci-Labware). A schematic of the electrochemical

cell is illustrated in Figure 3.12.

Figure 3.11 - Dimensions of cell envelope.

3. Experimental 48

Figure 3.12 - Electrochemical cell schematic.

Two different electrolytic cells were used, the schematic for each is presented in

Figure 3.13 and the cell head arrangements in Figure 3.14. A K-type thermocouple

(Omega), placed inside a one end closed glass tube was always immersed in the

salt to monitor the temperature.

For the fluidised cathode set-up, the cathode comprises particles of metal oxide that

are agitated/suspended in the salt eutectic melt, and a pure metal rod current

collector, that has a glass sheath around part of it so that 4 cm of it were always

immersed in the salt, taking into account the movement of the surface of the melt

as the salt was being agitated. The anode was separated in its own compartment to

avoid reoxidation of the reduced particles. A glass frit (porosity 2, 15 mm in

diameter, VWR International) was used to allow for the flow of salt ions. This frit

was fitted with a glass tube that contained the anode. The anode compartment has

an opening at the top to allow for gases to escape. The particles in the crucible

were agitated via the flow of argon (99.998% purity, BOC) through the melt. It was

bubbled via a ceramic tube (5 mm internal diameter, Alsint).

3. Experimental 49

Figure 3.13 - Electrolytic cells. (a) For a typical cell set-up for the electrochemical

reduction of thin films or metal cavity electrodes (MCEs), (b) for the electrochemical

reduction of metal oxide powder using the fluidised cathode method.

3. Experimental 50

Figure 3.14 - Ceramic cell heads hole arrangements. (a) For a typical cell set-up. (b)

For a fluidised cathode set-up. 1 is the argon inlet, 2 is the argon outlet, 3 is for the

reference electrode, 4 is for the working electrode or current collector (also used for

the sacrificial electrode in pre-electrolysis), 5 is for the thermocouple, 6a for the

anode, and 6b for the anode compartment containing the anode.

3.3.3 Electrodes

The electrode materials needed to possess mechanical integrity, not contaminate

the salt and withstand the relevant temperatures. They were immersed in the salt

eutectic melt and held in position via the silicone suba-seals. The suba-seals are

electrically insulating, and seal the cell from the outside environment. A three

electrode system was employed as required, as it provides the same frame of

reference for all the electrochemical experiments involving potential

measurements. All electrodes were 32-35 cm in length.

3.3.3.1 Reference electrode

The reference electrode used was a Ag/Ag+ electrode [121]. Borosilicate glass

tubes were used as the membrane sheath for the electrode; they have one closed

end and one open end with 3 mm internal diameter and a 1 mm wall thickness (to

aid transport). Silver wire with a 0.203 mm diameter (99.95% metal basis, Alfa

Aesar) was used. A silicone suba-seal was used to seal off the open end of the glass

tube, Figure 3.15.

The preparation of the reference electrode was conducted inside an argon glove

box. Initially, the glass tube was baked for a few hours, at 200 ˚C, to get rid of any

3. Experimental 51

residual moisture. Using treated dry salt, prepared as detailed in Section 3.3.4, 10 g

of LiCl-KCl eutectic was mixed with 0.1 g AgCl (>99% purity, Sigma Aldrich).

The mixture was then heated until molten to ensure homogeneity. After cooling

down, the 1wt% AgCl in LiCl-KCl was ground to fine powder. The silver wire was

then placed inside the glass tube and pushed all the way to the bottom. Using a

small funnel, 1 g of the 1 wt% AgCl in LiCl-KCl was put inside the glass tube. The

tube was then sealed using a silicone suba-seal and the top of the silver wire was

pushed through it, exposing it for electrical conduction. A cross-section of the

reference electrode showing its configuration is illustrated in Figure 3.15. When

running an experiment, the reference electrode was immersed in the molten LiCl-

KCl eutectic, the salt inside the tube also melts and the reference electrode is

operational. Whenever Ag/Ag+ electrode is mentioned in this thesis, it contains 1

wt% AgCl.

Figure 3.15 - Cross section of the Ag/Ag+ reference electrode.

When electrochemical measurements are taken, the silver wire reacts with the

chloride salt in the melt inside the glass membrane, according to the reversible

redox reaction in Equation 3.4, creating a standard potential reference.

𝐴𝑔𝐶𝑙 + 𝑒− ⇌ 𝐴𝑔 + 𝐶𝑙− 3.4

3.3.3.2 Counter electrode

A high density graphite rod was used as a counter electrode, 3.05 mm in diameter

(99.9995% metal basis, Alfa Aesar). The graphite rods were reusable if good

mechanical integrity were maintained. They were cleaned thoroughly with acetone

3. Experimental 52

before each use, and were heated via torch flame until glowing red, to remove any

residual water from them.

3.3.3.3 Working electrode

All metal oxide powders used, WO3 and UO2, are described in Chapters 5 and 6

respectively. Their characterisation is included as well. Three types of working

electrodes were used; thin film electrodes, metallic cavity electrodes, and fluidised

cathodes. They are summarised as follows.

a) Thin film electrode

A thin layer of oxide was thermally grown on a tungsten rod, 1.5 mm in diameter

(99.95% metal basis, Alfa Aesar). This was done by thermally oxidising the

electrode in air at a temperature ramp rate of 250 °C h-1

, then holding the

temperature at 700 °C for 2 h. The rod’s surface became yellow/green in colour,

indicative of the formation of WO3.

b) Metallic cavity electrode (MCE)

Metallic cavity electrodes [122-126] were made using a 0.5 mm thick molybdenum

sheet (Sigma-Aldrich, 99.9% purity). The sheet was cut into 5 mm wide and 6 cm

long strips. Holes, or cavities, of 0.4 mm diameter were drilled at one end of the

strips, as shown in Figure 3.16 (b). These strips were then attached to a tungsten

rod using a molybdenum wire. When experiments were run, it was ensured that

only half of a strip was immersed in the molten salt, with all the cavities exposed to

the electrolyte, but keeping the wire and the tungsten rod above it. Before

conducting electrochemical tests, the cavities were filled with fine powder of the

desired metal oxide to be reduced, using two glass slides and a ‘finger pressing’

technique (here, a glass slide was placed on the desk, some powder was place on

top of it, the MCE was places on top of the powder allowing it to fill the cavaties,

some more powder was placed on top of the MCE, then a glass slide on top.

Carefully using fingers, the two glass slides were pressed together to fill the

cavities with dense powder).

3. Experimental 53

Figure 3.16 – (a) Working electrode for fluidised cathode set-up. (b) Metallic cavity

working electrode (MCE).

c) Fluidised cathode

For the fluidised cathode set-up, the cathode compartment comprises metal oxide

particles that were suspended in the fused melt via bubbling argon, and a current

collector. The current collector was a tungsten rod, 1.5 mm in diameter (99.95%

metal basis, Alfa Aeser) that has a Pyrex sheath around it, Figure 3.16 (a), to make

sure that 4 cm of the electrode is always, and only, immersed in the salt during

experiments. This is to guarantee that all current response effects are due to

particle-electrode interactions, and not to the electrolyte’s surface vibrating through

agitation.

3.3.4 Electrolyte and cell atmosphere handling

All preparation steps were carried out under a sealed argon atmosphere, in an argon

glovebox (MBRAUN), where O2 levels were always kept at less than 0.5 ppm, and

water levels were also kept at less than 0.5 ppm. Anhydrous lithium chloride (ACS

reagent, ≥99.0% purity, Sigma-Aldrich) and potassium chloride (≥99.5% purity,

Sigma-Aldrich) were used for the electrolyte. The salt was dried in a vacuum oven

at 200 ˚C for 24 h to get rid of any residual moisture, then 59-41 mol% LiCl-KCl

3. Experimental 54

were mixed together. The salt was placed in the crucible, which was placed inside

the envelope, then, with the electrodes in place, the cell was sealed. This was then

transferred to the rig, to be heated, and the argon inlet and outlet were connected.

During experiments, a constant stream of argon was applied, creating a blanket at

the top of the reaction vessel.

Cyclic voltammetry measurements were carried out on a LiCl-KCl salt eutectic at

450 °C using a pure tungsten electrode as the working electrode, a graphite rod as

the counter electrode, and a Ag/Ag+ reference electrode. The scan rate applied was

50 mV s-1

, scanned from 0 V to -3 V to 3 V and back to 0 V. The cyclic

voltammogram produced is shown in Figure 3.17. It illustrates the potential

window of the LiCl-KCl salt eutectic. As can be seen, there are no significant

current peaks between the potentials 1.1 V and -2.6 V. Thus, between these two

potential values lies the salt’s potential window. At 1.1 V chlorine gas evolution

starts and it reaches its maximum level at about 2.6 V. At -2.6 V lithium metal

formation starts. From predominance diagrams of LiCl-KCl eutectics, Chapter 4,

one can deduce that lithium metal formation starts before potassium metal

formation. There is a peak on the left hand side of the diagram, in the positive

direction, this is due to the fact that when the potential is swept in the positive

direction lithium is oxidised back into its ionic form. This does not appear on the

right hand side of the graph, where chlorine gas evolution takes place, as the Cl2

molecules evolved leave the melt. Again, if the potential window of LiCl-KCl from

this cyclicvoltammogram is compared to that established in the predominance

diagram, noting that here the potential is shifted to the right, as it is against a

Ag/Ag+ reference electrode and the predominance diagram is against the standard

chlorine electrode, one can see that the width of the window is about the same, 3.7

V. This supports the validity of the predominance diagram for predicting the

stability of the electrolyte. As for most molten salts, the potential window of the

LiCl-KCl eutectic is very wide, which allows for flexibility in conducting

electrochemical reactions.

3. Experimental 55

Figure 3.17 - Cyclic voltammogram showing the potential window of LiCl-KCl

eutectic at 450 °C, scan rate 50 mV s-1

, and reference electrode: Ag/Ag+.

Figure 3.18 - Chronoamperogram showing a typical pre-electrolysis of LiCl-KCl

eutectic at 450 °C, set voltage: -2.300 V, and reference electrode: Ag/Ag+.

3. Experimental 56

Figure 3.19 - X-ray diffraction spectrum (Mo Kα) of treated LiCl-KCl salt, showing

KCl (165593-ICSD [127]), and LiCl (26909-ICSD [128]).

Prior to conducting electrochemical experiments, the electrolyte is treated further,

via applying a constant potential of -2.3 V, close to the salt’s decomposition

potential, using a sacrificial tungsten electrode, a pre-electrolysis step, to remove

any residual contaminant and moisture. This is usually applied for a few hours.

Figure 3.18 illustrates a typical pre-electrolysis chronoamperogram. The salt

eutectic was analysed via X-ray powder diffraction, Figure 3.19, which shows the

spectra of pure LiCl and KCl, indicating that the salt is contaminant-free (free of

metal ions).

3.3.5 Electrochemical set-up

A PC computer controlled potentiostat (IviumStat, Ivium Technologies, NL) was

used to perform electrochemical tests. Most experiments were run using a four lead

option, working electrode, counter electrode, reference electrode and ‘sense’. The

sense lead is attached to the electrode just below the working electrode’s lead to

reduce the voltage drop effects from the connecting cable, which might affect the

3. Experimental 57

potential difference between the working and the reference electrode. The

potentiostat was used to run electro-analytical tests such as cyclic voltammetry

(CV), linear sweep voltammetry (LSV), chronoamperommetry and

chronopotentiommetry.

3.4 Summary

In this chapter, the electrochemical and material characterisation techniques used in

this research have been outlined. The experimental designs and materials used have

also been covered, including the electrolytic cells, the electrodes and the salt

eutectic used in experiments. The procedures to prepare experiments and to treat

the LiCl-KCl eutectic have been outlined. Characterisation of the treated salt has

also been included.

4. Predominance Diagrams 58

4. Predominance Diagrams

Predominance phase diagrams for metal-molten salt systems are diagrams of

potential vs. the negative logarithm of the activity of O2-

ions (E-pO2-

); they are a

valuable tool for predicting and understanding electrochemical systems and for

optimising process conditions. Here, predominance diagrams are produced for U,

Pu, Np, Am, Cm, Cs, Nd, Sm, Eu, Gd, Mo, Tc, Ru, Rh, Ag and Cd species, in both

LiCl-KCl at 500 ˚C and NaCl-KCl at 750 ˚C. The two salt eutectics were chosen as

they are the two main systems for pyroprocessing; temperatures were selected

within each salt’s normal operating range. All of the diagrams presented show

regions of stability for the different metal species, their oxides and chlorides at unit

activity; however, this activity can be altered in accordance with the equations

derived. Examples of selective electrochemical reduction are also demonstrated for

potential spent fuel reprocessing in both salt systems.

4.1 Introduction

Different process schemes and salts have been studied in pyroprocessing, most of

which have been summarised by the NEA [15]. There are currently two main

molten salt technology processes in existence, both using chloride salts as

electrolytes; one in the US at the Argonne National Laboratory (ANL) using LiCl-

KCl eutectic for metallic fuel in integral fast reactors (IFR), and one in Russia at

the Research Institute for Atomic Reactors (RIAR) using NaCl-KCl eutectic for

oxide fuel for the Fast Breeder Reactor (FBR) [16]. There have also been other

advances made in molten salt nuclear pyroprocesses in Europe and Japan [19-24].

Predominance diagrams (also known as Littlewood diagrams) were originally

developed by Littlewood in 1962 [129] to summarise thermodynamic

characteristics of metal-molten salt systems and are akin to Pourbaix diagrams,

which describe the behaviour of species in aqueous solutions. In Pourbaix

diagrams, the equilibrium potential is plotted against pH to show regions of


stability for a specific system [130]. Predominance diagrams show potential vs. the

negative logarithm of the activity of oxygen anions: the pO2-

.

The fundamental notion of predominance diagrams is to represent thermodynamic

data in a diagrammatic form. Data is converted to linear equations relating free

energies to logarithmic functions of material composition. The free energies of the

system are represented by potentials that are relative to the electrochemical

equilibrium between the salt’s anion and its elemental form, in chloride salts this is

the standard chlorine electrode. The composition variable is the negative logarithm

of the activity of the oxygen ions in the melt, pO2-

(analogous to pH). These

diagrams are very useful when studying complex metal-molten salt systems, such

as Li-K-U-O-Cl, as they describe the regions of stability and the pathways of the

system’s evolution at different equilibrium potentials.

The procedure for producing predominance diagrams is best described in the works

of Littlewood, Dring et al. and Brown et al. [129, 131, 132]. Predominance

diagrams have been developed for various systems [133-140], providing valuable

insight into the electrochemical processes of metals and their oxides in molten

salts. However, they are not flawlessly accurate. Fundamentally predominance

diagrams are only as accurate as the thermodynamic data used to create them, and

therefore they need to be validated experimentally, as reaction kinetics and surface

processes will have a dominant effect in defining reaction rate. Extensive work has

been performed to characterise the reduction of TiOx in CaCl2; this has been

compared with the predominance diagram produced by Dring et al. [131]. X-ray

diffraction analysis of the reduction process of TiOx at different stages has been

carried out by Schwandt and Fray, Wang and Li, and Bhagat et al. [47, 50, 141],

providing validation of processes predicted by the predominance diagram.


Table 4.1 - List of nuclides commonly considered in burn-up credit criticality analyses

(from NEA 2011) [142] and used as the basis for the systems examined here.

Nuclide

Half-life (years)

Content in spent UOX

PWR fuela (g/MTHM)

52 GWd/t at discharge

Relative importance rank

for 40 GWd/MTHM PWR

fuel – 5 years coolingb 234U 2.45105 143 24 235U 7.04108 6050 1 236U 2.34107 5650 11 238U 4.47109 927000 3 238Pu 87.74 372 22 239Pu 2.41104 5810 2 240Pu 6550 2840 4 241Pu 14.40 1820 5 242Pu 3.76105 1020 19 237Np 2.14106 811 14 241Am 432.60 228 10 243Am 7370 1.74 11 243Cmc 28.50 0.624 244Cmc 18.11 141 245Cmc 8532 11 133Cs Stable 1630 12 143Nd Stable 1070 7 145Nd Stable 989 17 147Sm 1.061011 196 20 149Sm 2.001015 3.36 6 150Sm Stable 446 23 151Sm 93 14.7 9 152Sm Stable 134 15 153Eu Stable 184 18 155Gd Stable 3.93 13 95Mo Stable 1180 21 99Tc 2.10105 1120 16 101Ru Stable 1210 26 103Rh Stable 540 8 109Ag Stable 119 25 113Cdc 9.101015 a

Measured content from ARIANE experimental programme data. b

Based on relative sensitivity coefficients from the Nuclear Regulatory

Commission (2008) [143]. c Important for MOX fuel only.

Another limitation is that there is no adequate experimental technique for keeping

the O2-

levels within a specified limit. Studies have looked at monitoring and

controlling the pO2-

in molten salts [144]; but this is difficult to implement in

practice, particularly on a larger scale. Rigorous salt preparation can reduce the O2-

ion concentration; however, once the reduction of a metal oxide proceeds, the

increase of O2-

concentration in the melt shifts the equilibrium potential. Also, the


degradation and the formation of pores on the working electrode can act to ‘trap’

O2-

ions, thus increasing the levels at the interface, changing the local equilibrium.

New reactor cell designs could prove useful here, such as the fluidised cathode

process, as it would help disperse the O2-

ions, and give the system more

homogeneity.

The nuclides commonly considered in burn-up credit analysis by the NEA [142]

are shown in Table 4.1. This also lists their half-lives, their content (in g per metric

tonne of heavy metal, g/MTHM) in uranium oxide (UOX) pressurised water

reactor (PWR) spent-fuel and their relative importance according to the NEA. The

different nuclides of uranium and plutonium are of highest importance, as their

content in spent-fuel is significant. All of these nuclides are classed as high-level

waste.

In this chapter, predominance diagrams are produced for spent nuclear materials in

LiCl-KCl eutectic at 500 ˚C, as is in use at the ANL, and NaCl-KCl eutectic at 750

˚C, as is in use at the RIAR. Each temperature was chosen to be within the salts’

common operating temperature ranges [16, 35].

4.2 Theory

To construct a predominance diagram, one needs to employ the following

equations and follow the subsequent steps. The Gibbs energy of formation of a

given reaction, ΔG, may be related to the electrochemical potential of the specific

cell, E, by Equations 4.1 (away from standard state) and 4.2 (standard state), and

by the Nernst Equation 4.3. In these equations, n is the moles of electrons

transferred in a reaction, F is Faraday’s constant (96485.4 C mol-1

), R is the

universal gas constant (8.314 J mol-1

K-1

), T is the operating temperature of the

molten salt in Kelvin, and Q is the reaction quotient, a function relating the

activities of the different chemical species involved in a chemical reaction.

∆𝐺𝑜 = −𝑛𝐹𝐸𝑜 4.1

∆𝐺 = −𝑛𝐹𝐸 4.2

𝐸 = 𝐸𝑜 −𝑅𝑇

𝑛𝐹𝑙𝑛𝑄 4.3


The potential window of the salt defines the bounds of the predominance diagram.

For example, in LiCl-KCl, Cl2 gas evolution defines the oxidative limit, and

formation of metallic Li or K defines the reductive limit. The oxidative limit (in

this example, the formation of Cl2) is assigned a Gibbs energy of formation of zero,

at all temperatures, to maintain a zero potential reference point. This is when the

Cl2 formed is at unit activity with the Cl- ions in the melt at one atmospheric partial

pressure. From this zero reference point, the regions of stability for the system

chosen extend to the reductive limit, the decomposition potential of the salt.

There are three types of equilibrium that define the thermodynamic regions of

stability for a specific species: one that does not involve transfer of electrons; one

that does not involve transfer of oxide anions; and one that depends on both

electron and oxide anions being transferred. Equations 4.4, 4.5 and 4.6 represent

the three kinds of equilibrium reactions, where M is metal. Using Equations 4.1,

4.2 and 4.3, the equations in terms of electrode potential, E, and pO2-

, for each

reaction can be derived, as seen in Equations 4.7, 4.8 and 4.9.

𝑀𝑂𝑦 + 2𝑦𝐶𝑙− ↔ 𝑀𝐶𝑙2𝑦 + 𝑦𝑂

2− 4.4

𝑀(𝑧+𝑛)+ + 𝑛𝑒− ↔ 𝑀𝑧+ 4.5

𝑀𝑂𝑥 + 𝑛𝑒− ↔ 𝑀𝑂𝑥−𝑦 + 𝑦𝑂

2− 4.6

𝑝𝑂2− =∆𝐺4.4

𝑜 +𝑅𝑇𝑙𝑛(𝑎𝑀𝐶𝑙2𝑦

𝑎𝑀𝑂𝑦)

𝑦𝑅𝑇𝑙𝑛10 4.7

𝐸4.5 =−∆𝐺4.5

𝑜

𝑛𝐹−𝑅𝑇

𝑛𝐹𝑙𝑛 (

𝑎𝑀𝑧

𝑎𝑀𝑧+𝑛) 4.8

𝐸4.6 =−∆𝐺4.6

𝑜

𝑛𝐹−𝑅𝑇

𝑛𝐹𝑙𝑛 (

𝑎𝑀𝑂𝑥−𝑦

𝑎𝑀𝑂𝑥) +

𝑦𝑅𝑇𝑙𝑛10

𝑛𝐹𝑝𝑂2− 4.9

All thermodynamic data used is presented in Table 4.2.

Predominance diagrams for metal-molten salt systems are generally divided into

three main regions of stability, where different species exist: one where the pure


metal exists, either in the salt solution or precipitated as a solid; one where the

metal is covered by an oxide layer or is fully oxidised, also as a solute in the liquid

salt or as a solid species; and one where the metal is in the form of a chloride,

liquid or gas [129]. This is illustrated in Figure 4.1.

Figure 4.1 - Example of the three main regions of stability in a predominance diagram

(based on a LiCl-KCl salt eutectic) and showing the nature of reactions leading to

horizontal, vertical and diagonal lines.

The limiting potentials for the salt, at any given partial pressure of chlorine gas,

and activity of liquid lithium metal and liquid potassium metal, may be calculated

according to their corresponding Nernst Equations 4.13, 4.14 and 4.15, for the half-

cell reactions presented in Equations 4.10, 4.11 and 4.12.

𝐶𝑙2 + 2𝑒− ↔ 2𝐶𝑙− 4.10

𝐿𝑖+ + 𝑒− ↔ 𝐿𝑖 4.11

𝐾+ + 𝑒− ↔ 𝐾 4.12


𝐸𝐶𝑙2 = 𝐸𝐶𝑙2𝑂 +

𝑅𝑇𝑙𝑛10

2𝐹log(

𝑃𝐶𝑙2−

𝑃𝐶𝑙2) 4.13

𝐸𝐿𝑖 = 𝐸𝐿𝑖𝑂 +

𝑅𝑇𝑙𝑛10

𝐹log(

𝑎𝐿𝑖+

𝑎𝐿𝑖) 4.14

𝐸𝐾 = 𝐸𝐾𝑂 +

𝑅𝑇𝑙𝑛10

𝐹log(

𝑎𝐾+

𝑎𝐾) 4.15

For the LiCl-KCl electrolyte at 500 ˚C, the potentials are solid horizontal lines at 0

V, -3.57 V and -3.76 V for the evolution of Cl2, Li and K respectively. The same

procedure was applied to the NaCl-KCl eutectic at 750 ˚C, and the potentials for

the evolution of Na and K were found to be -3.28 V and -3.52 V respectively.

The next step is to calculate the Gibbs energy of formation of an oxide ion in the

melt. There are two standard states for the oxide ion in the salt eutectic, these are

Li2O and K2O. Using ΔG˚ of Equations 4.11 and 4.12, and ΔG˚ of the following

half-cell reactions 4.16, 4.17 and 4.18, one can calculate the Gibbs energy of

formation for the O2-

ion.

2𝐿𝑖 +1

2𝑂2 ↔ 2𝐿𝑖+ + 𝑂2− 4.16

2𝐾 +1

2𝑂2 ↔ 2𝐾+ +𝑂2− 4.17

1

2𝑂2 + 2𝑒

− ↔ 𝑂2− 4.18

At 500 ˚C, the Gibbs energy of formation for O2-

from Li2O is +191.71 kJ mol-1

,

and that from K2O is +469.32 kJ mol-1

. The salt in this system is a LiCl-KCl

eutectic with a molar composition of 59-41 respectively. Therefore, the assumed

Gibbs energy of formation for the oxide ion at 500 ˚C in this system was calculated

as +305.56 kJ mol-1

. The same method was applied to the NaCl-KCl equimolar

system at 750 ˚C, and the Gibbs energy of formation for the oxide ion in the molten

salt was established to be +408.91 kJ mol-1

.

Moreover, oxygen anions are in equilibrium with the atmosphere. Using the Gibbs

energy of formation for O2-

and the Nernst equation, Equation 4.19 can be derived.


𝐸𝑂2− = 𝐸𝑂2−𝑜 +

𝑅𝑇𝑙𝑛10

4𝐹𝑙𝑜𝑔𝑃𝑂2 +

𝑅𝑇𝑙𝑛10

2𝐹𝑝𝑂2− 4.19

This shows the relationship between the electrode potential as a function of

standard state potential, the oxygen gas partial pressure and the negative logarithm

of the oxide ion activity, pO2-

. It is depicted in the predominance diagram in Figure

4.2 as a series of diagonal dashed lines, showing several states of equilibrium

between oxygen gas and oxide ions at various O2 partial pressures. The information

from this diagram is useful for molten salt systems, where partial pressures of

oxygen are normally very low, as it shows the potentials at which O2 would be in

equilibrium with the melt. These lines can be derived for the NaCl-KCl eutectic

system following the same procedure. They can be superimposed onto the metal

phase diagrams; however, to aid visual interpretation this is not performed here.

Nonetheless, it is shown for uranium species, in Figure 4.9 and Figure 4.10.

Figure 4.2 - Predominance diagram for the Li-K-O-Cl system at 500 °C, illustrating

the relationship between oxygen pressure, oxide activity and potential, E, relative to

the standard chlorine electrode.


Table 4.2 - Gibbs energy of formation for nuclear materials at 500 °C and 750 °C.

Species

ΔGf0

(kJ mol-1)

at 500 ˚C

ΔGf0

(kJ mol-1)

at 750 ˚C Comments

References

LiCl (NaCl) -344.887 (-316.766)

[145-147]

Li2O (Na2O) -498.105 (-274.608)

[145, 148-152]

KCl -362.418 -339.473

[153, 154]

K2O -255.559 -220.057

[148, 149]

Li2UO4 (Na2UO4) -1662.680 (-1486.881) Extrapolated above 27 ˚C [151, 152, 155]

UO -477.980 -455.771 Extrapolated above 227 ˚C [156]

UO2 -950.563 -907.756

[149, 157]

U4O9 -3921.203 -3736.848

[149]

U3O8 -3055.659 -2896.089

[149]

UO3 -1023.699 -962.404

[145, 148, 149]

UCl2O -888.723 -837.432 Extrapolated above 527 ˚C [150, 155]

U2Cl5O2 -1796.568 -1677.069 Extrapolated above 427 ˚C [151]

UCl3 -692.619 -642.160

[151, 155]

UCl4 -794.542 -735.413

[145, 158]

Pu2O3 -1475.278 -1410.029

[148, 149]

PuO2 -908.526 -861.327

[148-150]

PuClO -806.625 -765.969

[155, 157]

PuCl3 -787.684 -733.376

[154, 155, 158]

Li2NpO6 (Na2NpO4) -1355.089 (-1346.466) Extrapolated above 27 ˚C [152]

NpO2 -938.781 -894.564

[149, 155]

Np2O5 -1822.103 -1709.164 Extrapolated above 527 ˚C [155]

NpCl3 -725.188 -670.084

[155, 157]

NpCl4 -758.434 -705.146 Extrapolated above 727 ˚C [155]

Am2O3 -1473.270 -1406.104

[155]

AmO2 -795.968 -756.153

[147, 155]

AmClO -817.211 -777.668

[155]

AmCl3 -795.282 -739.928 Extrapolated above 27 ˚C [155]

Cm2O3 -1430.229 -1328.186 Extrapolated above 27 ˚C [159]

CmO2 -748.492 -684.109 Extrapolated above 27 ˚C [159, 160]

CmClO -807.035 -744.292 Extrapolated above 27 ˚C [159]

CmCl3 -767.028 -683.871 Extrapolated above 27 ˚C [159]

Cs2O -242.961 -205.878

[148, 158]

Cs2O2 -283.880 -228.116

[148, 156, 158]

Cs2O3 -327.724 -259.383 Extrapolated above 427 ˚C [150]

CsO2 -173.900 -136.486 Extrapolated above 200 ˚C [151]

Nd2O3 -1587.405 -1519.064

[148, 149, 151]


NdO2 -696.669 -447.885 Extrapolated above 200 ˚C [161]

NdClO -855.745 -811.395

[145]

NdCl3 -852.695 -796.445

[151]

Sm2O3 -1597.552 -1524.838

[145, 149]

SmClO -861.226 -817.884 Extrapolated above 727 ˚C [151]

SmCl3 -838.055 -783.450

[145, 158]

EuO -513.126 -489.177

[148, 151, 158]

Eu2O3 -1409.857 -1335.566

[151]

EuClO -750.346 -703.088

[156]

EuCl3 -737.401 -683.034

[151, 158]

Gd2O3 -1615.765 -1545.666

[148, 157, 158]

GdClO -841.390 -797.533 Extrapolated above 727 ˚C [145, 147, 148]

GdCl3 -815.177 -765.475 Extrapolated above 727 ˚C [150, 158]

Li2MoO4 (Na2MoO4) -1232.046 (-1086.773)

[149, 151, 154,

162]

MoO2 -445.688 -402.174

[149, 152, 163]

MoO3 -547.974 -487.662

[145, 148, 149]

MoCl2O -357.355 -305.131

[147, 148]

MoCl2 -172.364 -136.825

[147, 148, 151]

MoCl5 -284.750 -223.078

[146, 158]

TcO2 -314.411 -270.073

[150, 156]

TcO3 -349.485 -300.478 Extrapolated above 127 ˚C [145]

Tc2O7 -738.443 -644.378 Extrapolated above 427 ˚C [149, 154]

TcCl3 -133.223 -88.864

[156]

RuO2 -172.921 -133.825

[150]

RuO4 -47.246 0.188 Extrapolated above 172 ˚C [151]

RuCl3 -54.551 -4.138 Extrapolated above 450 ˚C [150]

Rh2O -57.798 -47.534 Extrapolated above 727 ˚C [157]

RhO -31.229 -13.954

[157]

RhCl -40.413 -28.786

[156]

RhCl2 -65.752 -41.129

[156]

RhCl3 -92.650 -39.953

[150-152]

Ag2O 19.033 33.200

[147, 151, 159]

AgO 55.057 76.216 Extrapolated above 125 ˚C [160]

Ag2O3 286.073 386.987 Extrapolated above 125 ˚C [159]

AgCl -86.475 -80.450

[147, 151, 152]

CdO -181.481 -155.461

[151, 152, 157]

CdCl2 -270.663 -240.187

[147, 150]


The functions of the interface lines between the different regions of stability, for

each system, were derived using Reactions and Equations 4.4-4.9 and the

thermodynamic data presented in Table 2. These are all presented in Appendix B.

Pure liquid phases were assigned an activity of unity and pure gases were assigned

unit atmosphere partial pressure. The diagrams depict regions of stability for

species in solution and solid phases that would precipitate out, as well as liquid and

gas phases. These diagrams can also be altered, by changing the activity used and

conditions, which would shift the equilibria lines accordingly. For simplicity, unit

activities are shown here. The predominance diagrams for the spent nuclear fuel

materials in LiCl-KCl eutectic at 500 ˚C and NaCl-KCl eutectic at 750 ˚C are

presented in Figure 4.3, Figure 4.4, Figure 4.5 and Figure 4.6.

4.3 Results

Some of the general features of the predominance diagrams can be illustrated by

reference to the uranium system in Figure 4.3(a). Compounds with uranium at their

highest oxidation states are found in the upper regions of the diagram, at less

negative potentials. At very low oxide ion activities, on the right hand side of the

diagram, uranium chlorides (UCl3, UCl4) are formed. Uranium metal can be

extracted from these at less negative potentials compared to the reduction of UO2.

Since there is no transfer of O2-

ions in these reactions, from U4+

to U3+

and to U,

the resultant interline functions between these species’ regions of stability are

independent of pO2-

. Thus, the interfaces are represented as horizontal lines in the

predominance diagram.

Due to the stability of other compounds that could exist in the melt, there is an

intermediate region of stability between the oxides and the chlorides. This

intermediate region is the metal oxychloride form. In the case of uranium it is

UCl2O and U2Cl5O2. The interface between these two phases is a horizontal line, as

there is no transfer of O2-

occurring. The interfaces between the regions of stability

of the reactions from UO2 to UCl2O and to UCl4 are all vertical lines; hence, the

derived interface equations are independent of potential, as the ratio of U


molecules does not change. All the other equations for interface lines related to

UCl2O and U2Cl5O2 are dependent on both pO2-

and E, resulting in diagonal lines.

At high oxide ion activities, on the left hand side of the predominance diagram,

other types of uranium oxides are predicted to form. This is due to the oxide ions

reacting with lithium or potassium ions in the fused salt. In the case of uranium, the

first stable metal uranium oxide to form is Li2UO4, as presented in Figure 4.3(a).

For the electrochemical reduction of UOx to U metal, the reductive potential is very

negative and close to the salt’s decomposition potential. The diagram gives an

indication of the number of phases expected to appear, and the order they would

appear in, if UO3 were to be reduced. It can also be deduced that some compounds,

such as U3O7, UO and U4O9, have a very narrow band of stability, when reducing

the uranium oxide; thus, it is not very likely that traces of them will remain in the

final product, as is the case with TiOx [50].

The predominance diagrams for U in NaCl-KCl, and for Pu, Np, Am, Cm, Cs, Nd,

Sm, Eu, Gd, Mo, Tc, Ru, Rh, Ag and Cd, in both LiCl-KCl and NaCl-KCl, can be

interpreted in the same way as for U in LiCl-KCl. They are all presented in Figure

4.3(a-h), Figure 4.4(a-h) Figure 4.5(a-h) and Figure 4.6(a-h).


Figure 4.3 - Predominance diagrams of (a) U, (c) Pu, (e) Np, and (g) Am in LiCl-KCl

at 500 °C. (b) U, (d) Pu, (f) Np, and Am are in NaCl-KCl at 750 °C.


Figure 4.4 - Predominance diagrams of (a) Cm, (c) Cs, (e) Nd, and (g) Sm in LiCl-KCl

at 500 °C. (b) Cm, (d) Cs, (f) Nd, and Sm are in NaCl-KCl at 750 °C.


Figure 4.5 - Predominance diagrams of (a) Eu, (c) Gd, (e) Mo, and (g) Tc in LiCl-KCl

at 500 °C. (b) Eu, (d) Gd, (f) Mo, and Tc are in NaCl-KCl at 750 °C.


Figure 4.6 - Predominance diagrams of (a) Ru, (c) Rh, (e) Ag, and (g) Cd in LiCl-KCl

at 500 °C. (b) Ru, (d) Rh, (f) Ag, and Cd are in NaCl-KCl at 750 °C.


4.4 Discussion

The interface lines between the lowest oxidation state and the pure metal state are

of particular importance for electrochemical reduction and pyroprocessing, for

metal fuelled reactors; and the interface line between oxide states for oxide fuelled

reactors. From Figure 4.3, Figure 4.4, Figure 4.5 and Figure 4.6, it is evident that

the reduction of the transition metals, Rh, Ag, Ru, Tc, Cd and Mo, showed first, at

less negative potentials. Then, the reduction of U, Pu and the minor actinides, in

the order of U, Cs, Pu, Am, Np and Cm, at increasingly negative potentials.

Finally, the reduction of the lanthanides in order of increasingly negative potential:

Nd, Sm, Gd and Eu. These findings are encouraging, as the three different groups

are clearly divided, thus indicating that they can be selectively reduced and

separated from the spent-fuel. Separating Pu as a single species appears to be

challenging, this is an important non-proliferation feature.

From the predominance diagrams, it is also evident that some species have very

narrow bands of stability, and thus might not be observed experimentally, if the

metal oxides were to be reduced. This could be the case for U3O7, UO, U4O9,

Np2O5, Cs2O2, MoCl2O, TcO3 and RhCl2.

4.4.1 Comparison of eutectics

It is noticeable from the predominance diagrams that for all spent fuel materials,

the region of stability for the pure metal phase is larger in the LiCl-KCl eutectic,

than in the NaCl-KCl eutectic, even though the temperature is lower, 500 ˚C and

750 ˚C respectively. The final reduction stage from oxide to metal also appears to

be possible at higher O2-

ion activities in LiCl-KCl compared to NaCl-KCl. Thus, it

seems that the use of LiCl-KCl for molten salt pyroprocessing is more favourable

than NaCl-KCl. However, this is purely based on thermodynamics and higher

temperatures are likely to benefit from faster kinetics.

4.4.2 Selective electro-reduction

Selective direct reduction is when there is a mixture of different metal oxides and

only one, or a subset of them, are selectively reduced and separated via

electroplating. This can be very beneficial in the nuclear industry, to enable the


reprocessing and recycling of the useful spent fuel products selectively and

separately, and also prevents nuclear proliferation. Thus, it is important to

understand how control of electrode potential affects spent fuel products in a

molten salt reprocessing reactor; comprehensive predominance diagrams are a

starting point for such analyses.

One instance where the possibility of selective electro-reduction is not desired is

for U and Pu. This is for anti-proliferation reasons. The predominance diagrams for

Pu are superimposed onto those for U, in LiCl-KCl and NaCl-KCl, Figure 4.7. The

main region of interest is the interface lines where the reduction from the lowest

oxide state to the pure metal phase occurs (e.g. from -3 V and 10 pO2-

to -3.5 V and

11 pO2-

in Figure 4.7(a)). As seen in Figure 4.7, these boundary lines for the U and

the Pu systems are very close to each other. Thus, it would be challenging to

selectively reduce one of them exclusively. From Figure 4.7 it is evident that the

uranium oxide reduction to uranium metal would occur first.

In principle, the use of predominance diagrams to aid the understanding of the

feasibility of selective electro-reduction can be applied to all spent-fuel products, in

order to understand the reduction procedure that they would undergo if they were

all placed in a molten salt reprocessing reactor. Within each group, transition

metals, actinides and lanthanides, difficulties are apparent in the partial reduction

of a single species. In the transition metals group, partial reduction is the most

likely to be achieved, with the exception of Cd and Mo, as their reduction interlines

are too close to each other. In the actinides group, it is more difficult to perform

partial reduction, Np being the exception. It is the most difficult to carry out partial

direct reductions in the lanthanides group.


Figure 4.7 - Predominance diagrams of U and Pu species. (a) in LiCl-KCl at 500 °C,

(b) in NaCl-KCl at 750 °C.

An example of where selective direct reduction would be useful is for Am and Cm:

this is to mimic the EXAm [164] process in solvent extraction nuclear

reprocessing. Am and Cm are both high heat emitters; however, Cm has a short

half-life, ~ 18 years, and thus, it is not economically viable to reprocess it, as

storage options would be cheaper. By contrast, it is desirable to reprocess and

recycle Am. The predominance diagrams for Cm are superimposed onto those for

Am, in LiCl-KCl and NaCl-KCl, Figure 4.8. From the predominance diagrams, it is

evident that selective electro-reduction would be difficult for these two species due

to the similarity in reduction potential. However, in this instance, the use of NaCl-

KCl eutectic melt as the electrolyte is more favourable as the potentials for

reduction of the two species form a larger window.

Figure 4.8 - Predominance diagrams of Am and Cm species. (a) in LiCl-KCl at 500 °C,

(b) in NaCl-KCl at 750 °C.


4.4.3 Effect of temperature

Temperature is an important parameter that affects the stability zones. At higher

temperatures the regions of stability are shifted to the ‘left’, at higher O2-

activity,

where oxide reduction reactions occur; however, at higher temperatures, the

potential window is smaller. Figure 4.9 and Figure 4.10 present predominance

diagrams for uranium species in LiCl-KCl eutectic at 500 °C and 800 °C

respectively. When comparing the two diagrams, one can clearly see the effects of

temperature on the regions of stability. An interesting feature is also the

disappearance of the stability band for UO entirely at the higher temperature,

indicating that what could be a very fast 2 step 4-electron transfer process from

UO2 to U becomes a one step process.

From a design point of view, processes at lower temperatures are favourable, as

heat requirements would be lower, also, the materials designated for reactor

vessels, sealing, handling and maintenance are more practical at lower

temperatures. Thus, provided that the fused salt is treated and O2-

levels are kept

minimal, it is constructive to keep the operating temperatures low. These processes

are also possible at lower temperatures as shown in Figure 4.9.

4.5 Conclusions

Predominance diagrams are useful tools for understanding the electrochemistry and

phase stability of metal-oxide systems in molten salts. They help in predicting

experimental results and give a good indication of whether an electrochemical

process is thermodynamically feasible. Predominance diagrams have been

generated for the range of spent nuclear fuel materials, based on established

thermodynamic properties of the materials published in the literature.

By superimposing the diagrams for U and Pu, and Am and Cm onto each other, the

potential for selective electro-reduction can be determined and the most suitable

molten salt and temperature selected. It was found that molten salt pyroprocessing

provides a promising route for reprocessing nuclear materials, which is also

proliferation resistant. However, the approach is not seen to be a replacement for

the EXAm process, as the selective reduction and separation of Am species from

Cm would be challenging due to similarity in reduction potential.


Figure 4.9 - Predominance diagram for the Li-K-U-O-Cl system at 500 °C.

Figure 4.10 - Predominance diagram for the Li-K-U-O-Cl system at 800 °C.

5. The Electrochemical Reduction of Tungsten Oxide 79

5. The Electrochemical Reduction of Tungsten Oxide

The electrochemical reduction of WO3 to W metal using a fluidised cathode

process was investigated. Voltammetry studies were conducted, and alongside a

predominance diagram that was constructed, the reaction path-way was studied.

The main reduction potential appeared to be -2.14 V. A complete reduction,

through applying constant voltage, was achieved with a Faradaic current efficiency

of ~ 82%. The metal product is in the form of particles, either deposited on the

electrode surface or settled at the bottom of the crucible. The effects of metal oxide

– salt ratio and of fluidisation rate on the process were also investigated. Higher

loading of metal oxide particles resulted in an increase in the rate of deposit

growth, and in a decrease in particle-current collector collision-reaction spikes. An

increase in fluidisation rate resulted in an increase in both rate of growth of deposit,

as well as collision-reaction noise.

5.1 Introduction

Tungsten has many applications because of its physical and chemical properties

[165]; this has sparked renewed interest in its production in the U.K. recently.

Tungsten ore is usually extracted then converted to WO3, which is in turn reduced

to W metal by hydrogen and heating. This process is time and energy intensive

[166]. The electrochemical reduction route might prove to be viable for the

production of pure tungsten. Studies on the electrochemical reduction of tungsten

oxide in molten salts have been published [167-169], mainly for the production of

metal alloys [59, 170]. The precursors were porous solids in the form of pellets.

This Chapter reports on the current efficiency and the investigation of different

characteristics that affect the fluidised cathode process, using a model system of

tungsten oxide and LiCl-KCl molten salt eutectic.


5.2 Experimental

5.2.1 Apparatus

A schematic of the electrolytic cell used is illustrated in Figure 3.13(b). The

cathode consists of WO3 particles that are suspended in the molten salt (LiCl-KCl)

eutectic, and a pure tungsten rod current collector. The anode is a graphite rod

separated in its own compartment to avoid reoxidation of the reduced tungsten

particles; the anode compartment has an opening at the top to allow for gases to

escape. The melt is agitated via a flow of argon. A reference electrode (Ag/Ag+)

was used, and the temperature was monitored via a thermocouple that is immersed

in the melt. All electrochemical tests were performed using a potentiostat

(IviumStat, Ivium Technologies, NL).

The same set-up was used to carry out the thin film experiments. However, no

WO3 particles were employed, the counter electrode was not separated inside a

compartment, no argon was bubbled through the melt, and the current collector was

replaced with a thin film cathode, Figure 3.13(a).

5.2.2 Chemicals

All preparation steps were carried out under a sealed argon atmosphere. Anhydrous

lithium chloride (ACS reagent, ≥99.0% purity, Sigma-Aldrich) and potassium

chloride (≥99.5% purity, Sigma-Aldrich) were used for the electrolyte. The salt

was dried in a vacuum oven at 200 ˚C for 24 h, then 150 g of 59-41 mol% LiCl-

KCl were mixed with WO3 (99.9% purity, Alfa Aesar). Particle size distribution

measurements, Figure 5.1, were conducted using a Beckman Coulter LS13320

laser diffraction particle size analyser. Figure 5.2 shows the x-ray diffraction

pattern of the as-received WO3 powder. 15 g of LiCl-KCl was placed inside the

anode compartment. The counter electrode was a high density graphite rod, 3.05

mm in diameter (99.9995% metal basis, Alfa Aesar). The working electrode

(current collector) was a tungsten rod, 1.5 mm in diameter (99.95% metal basis,

Alfa Aesar). A glass sheath around the shaft of the tungsten rod ensured that a

constant surface area of electrode is exposed to the electrolyte, even when being


agitated by the Ar stream. Argon (99.998% purity, BOC) was bubbled through the

melt via a ceramic tube (5 mm internal diameter, Alsint).

For the thin film experiments, the electrolyte was composed of 150 g of LiCl-KCl,

prepared in the same way as for the fluidised cathode setup, but no WO3 particles

were used. The working electrode was thermally oxidised in air at a temperature

ramp rate of 250 ˚C h-1

, then held at 700 ˚C for 2 h.

Figure 5.1 - Average particle size distribution of WO3 powder in terms of volume

percentage, also showing the maximum size distribution and the minimum, with a

standard deviation of 2. Mean particle diameter: 30.9 μm, median particle diameter:

29.5 μm.


Figure 5.2 - X-ray diffraction spectrum (Mo Kα) of as-received WO3 powder sample,

showing WO3 (1620-ICSD [171]).

5.2.3 Procedure

Experiments were carried out under a dry argon atmosphere at a melt temperature

of 450 ˚C (unless indicated otherwise). Argon was bubbled into the melt, which

resulted in a homogeneous distribution of particles as assessed by visual inspection.

The current collector, 4 cm length (area of 3.84 cm2), was immersed in the melt

during the fluidised cathode measurements, and 2 cm length (area of 1.96 cm2)

during the thin film measurements. However, absolute currents are reported due to

the fact that the electrode surface area changes during experiments.

5.3 Results and discussion

A predominance diagram, Chapter 4, was constructed for the Li-K-W-O-Cl system,

relating the potential E vs. standard chlorine electrode (S.Cl.E) to the negative

logarithm of O2-

ions activity, pO2-

. All the thermodynamic data used for the


production of the diagram is presented in Table 5.1, and the derived interface

equations are presented in Appendix C. A predominance diagram for tungsten

species in CaCl has been published [59]; however, this is the first in LiCl-KCl

eutectic.

Table 5.1 - Gibbs energy of formation at 500 °C for species in the Li-K-W-O-Cl

system.

Species ΔG0f at 500 ˚C

(kJ mol-1)

References

WO2 -447.23 [149, 150, 163]

WO3 -641.66 [150]

WCl2O2 -586.35 [146, 147]

WCl2 -165.85 [146, 147, 150]

WCl4 -237.11 [146, 150]

LiCl -344.89 [145, 147, 163]

Li2O -498.11 [145, 148, 149]

KCl -362.42 [153, 154]

K2O -255.56 [148, 149]

The predominance diagram in Figure 5.3 shows the different regions of stability for

different compounds and oxidation states of tungsten. Thermodynamically, one can

deduce that the concentration of O2-

ions in the eutectic melt does not hinder the

reduction process of WO3 to W. However, it affects the reaction pathway. Starting

with WO3, two reduction reactions take place to produce W metal. These are

presented in Equations 5.1 and 5.2. Equations 5.3 and 5.4 are used to calculate the

potential, E, needed for each reaction to take place, at different values of O2-

ion

activity.

𝑊𝑂3 + 2𝑒− ↔𝑊𝑂2 + 𝑂

2− 5.1

𝑊𝑂2 + 4𝑒− ↔𝑊 + 2𝑂2− 5.2

𝐸5.1 =−∆𝐺5.1

0

2𝐹+𝑅𝑇𝑙𝑛10

2𝐹𝑝𝑂2− 5.3

𝐸5.2 =−∆𝐺5.2

0

4𝐹+2𝑅𝑇𝑙𝑛10

4𝐹𝑝𝑂2− 5.4


Figure 5.3 - Predominance diagram for the Li-K-W-O-Cl system at 500 °C.

The bands of potentials and pO2-

values are presented in Table 5.2. When

comparing features in the predominance diagram, such as the evolution of Li or

Cl2, with when they appear in the cyclic voltammogram in Figure 3.17, the

difference in potential between the Ag/Ag+ reference electrode and the S.Cl.E is

1.136 V. The potentials needed for the proceeding of Equations 5.1 and 5.2, with

respect to Ag/Ag+, are also presented in Table 5.2.


and potentials required for

Equations 5.1, 5.2, 5.6 and 5.7 to take place.

Reaction pO2- E (V vs. S.Cl.E) E (V vs. Ag/Ag+)

5.1 0.000 – 24.411 -2.591 – -0.721 -1.455 – +0.415

5.2 0.000 – 24.575 -2.740 ̶ -0.860 -1.604 – +0.276


Voltammetry measurements were performed on a thermally grown thin film of

WO3 electrode set-up and a fluidised cathode set-up, Figure 5.4 (a) and (b)

respectively. Both were scanned from 0 V to -2.7 V, and back to 0 V (vs. Ag/Ag+

reference electrode). The peaks at 2 represent the reduction of Li+, and at 3

represent its reoxidation. The smaller peaks appearing after 3 in the positive

direction represent the reoxidation of W to WOx. The peaks at 1 represent the main

reduction step in Equation 5.5.

𝑊𝑂3 + 6𝑒− ↔𝑊 + 3𝑂2− 5.5

When comparing these two peaks in Figure 5.4 (a) and (b), one can see that the

peak in (b) is shifted slightly to the right, and appears at -2.14 V, before peak 1 in

(a) which appears at -2.21 V, as indicated by the dashed vertical lines in the

diagrams. This is due to the nature of the fluidised cathode process, which provides

less resistance. For an electrochemical reaction, a 3PI needs to be present for it to

occur. In a molten salt system, this is the current collector (conductor), the fused

salt (electrolyte) and the metal oxide (insulator). A 3PI is instantly initiated at the

collision point of a particle with the current collector in the fluidised cathode

process, thus the reaction extends from this point. In the case of a thin film, there is

an insulating sheath of oxide covering the electrode surface, and therefore it is

harder for the reaction to take place, resulting in more driving force being required,

a higher overpotential. The same is true for peak 2.


Figure 5.4 - (a) Cyclic voltammogram of WO3 thin film cathode in LiCl-KCl eutectic

at 450 °C, scan rate: 50 mV s-1

, reference electrode: Ag/Ag+. (b) Cyclic voltammogram

of WO3 fluidised cathode in LiCl-KCl eutectic at 450 °C, scan rate 50 mV s-1

,

reference electrode: Ag/Ag+.


Figure 5.5 - Cyclic voltammogram of WO3 fluidised cathode in LiCl-KCl eutectic at

450 ˚C, scan rate: 50 mV s-1

, and reference electrode: Ag/Ag+.

Looking closer at the voltammetry of the fluidised cathode system, scanned from 0

V to -2.5 V and back to 0 V, Figure 5.5, provides a better understanding of the

process. The main reduction peak appears at -2.14 V again. The apparent ‘noise’ or

peaks in the voltammetry are due to particle-current collector collisions and

reactions. One can see that the collision-reaction features are only evident at

voltages where WO3 can be reduced; indicating that the reoxidation to WOx is

primarily a surface process involving W on the surface and not as a result of

reduced W particles colliding and reacting with the electrode. Looking closely at

the cyclic voltammogram, one can see that the electrochemical reaction proceeds

from actually -1.13 V, the noise in the diagram is also indicative of this. When

comparing the voltages involved with the thermodynamically calculated ones and

the predominance diagram, it suggests that the process starts with an

electrochemical reaction, Equation 5.1, which produces WO2. This then proceeds

to reduce electrochemically to W metal in a 4-electron-transfer step, Equation 5.2.


Usually, an electrochemical reduction process progresses vertically down the

predominance diagram; however, in reality it is much more complicated than that,

and it normally propagates to the left and down as the reactions resume. Previous

studies [55, 58, 59] where solid precursors have been removed from the melt mid-

reduction and analysed have shown that; in the case of tungsten reduction seems to

occur directly from WO3 to WO2 to W metal; and in the case of titanium reduction,

the perovskite CaTiO3 is formed in much smaller quantities than predicted by

predominance diagrams, at the beginning of the reduction process. This is due to

the fact that predominance diagrams fail to take into account the kinetics of

processes and the overpotentials involved in refractory metal production.

Figure 5.6 - Chronoamperogram of WO3 fluidised cathode in LiCl-KCl eutectic at 450

°C, 40 g WO3, argon flow rate: 800 cm3 min

-1, set voltage: -2.14 V, reference electrode

Ag/Ag+. 1 - 4: diagrammatic representation showing the evolution of the cathode's

surface area over time.


Figure 5.7 - (a) Photographs of current collector before the reduction process, 1, and

after, 2, (b) photograph of the solidified melt showing separate layers of WO3 and W.

Constant potential of -2.14 V, as identified as the reducing potential, (vs. Ag/Ag+

reference electrode) was applied to the fluidised cathode system, Figure 5.6, this

time 40 g of WO3 powder was fluidised. In previous studies [47, 50, 52, 170]

potentials in the range of 2 – 3 V were normally applied for such metal oxide

reduction processes. As time passed, the current increased due to an increase in the

electrode surface area and the deposit growth of W on the current collector surface,

as illustrated in Figure 5.6 (1-2). This growth is visible through the glass

electrolytic cell set-up. At (2) in Figure 5.6, there is a rapid reduction in current

associated with spalling off of the deposit from the electrode. New growth is then

associated with the increase in the current (3-4).

Due to the difference in densities, when the electrolyte is left to solidify, different

layers of materials form, Figure 5.7 (b). Dark metallic tungsten metal sinks to the

bottom, followed by a layer of green, still to be reduced, tungsten oxide, and finally

a layer of LiCl-KCl eutectic. This represents a potentially simple means of

separating the product in a technological system. Figure 5.7 (a) shows an image of

the current collector before, 1, and after the reduction, 2.

There are two areas from where the final product could be collected, the bottom of

the cell crucible, Figure 5.7 (b), and on the surface of the working electrode, Figure

5.7 (a). Samples from both were analysed using SEM and EDS (Carl Zeiss


XB1540, accelerating voltage = 20.00 kV, dwell time = 1.6 min). In the EDS

spectrum of the deposit’s surface on the working electrode, Figure 5.8, no oxygen

was detected. However, there is some salt present. Figure 5.9 (a) shows an SEM

image of the as-received WO3 particles and (b) that of the W product from the

solidified melt. No oxygen was detected in the EDS spectrum of the surface of the

W product.

A cross-section of the retrieved current collector, where the deposit did not spall

off, Figure 5.10, shows that the growth is cylindrical in shape, i.e. it forms at the

same rate perpendicular to the current collector surface in all directions. When

zoomed in, Figure 5.11, the particles of W metal produced appear homogeneous in

size and porosity.

Figure 5.8 - (a) EDS spectrum of the deposit on the current collector's surface. (b)

Photograph of the working electrode after the chronoamperometry showing the

deposited W. (c) SEM image of the cross-section of the solid W working electrode, 2,

and the deposit grown, 1.


Figure 5.9 – SEM image of the as-received WO3 particles, and (b) product W obtained

from the solidified melt as a separate layer to the LiCl-KCl. EDS spectra (penetration

depth ~6 μm) showed the stoichiometric composition of WO3 in (a), and the absence of

oxygen in (b).


Figure 5.10 - SEM image of the cross-section of the current collector showing the

deposited tungsten.

Figure 5.11 - SEM images of the current collector and deposit, at different

magnitudes, showing the homogeneity of reduced deposit.


5.3.1 Current efficiency

A constant potential of -2.14 V (vs. Ag/Ag+) was applied to the fluidised

cathode set-up to reduce 4 g of WO3 to W metal, Figure 5.12. The

chronoamperogram shows a similar trend to previously published work on

electrochemical reduction of metal oxides [58], where the diagram can be

segregated into two main sections; the first, where rapid reduction reactions

occur and the current increases significantly until it reduces; the second,

where the current increases slightly again, then plateaus and slower

reactions take place to reduce the final oxides, until the current decreases

indicative of the end of the process and the production of pure metal.

Assuming 100% current efficiency, it would require a charge of 9977.5 C to

be applied to fully reduce all 4 g of WO3. The XRD spectrum of the final

product, retrieved from the deposit on the current collector, is presented in

Figure 5.13. It shows the spectra of W and KCl. Most importantly, the

analyses show that there is no WO3, WO2 or any other quasi-reduced

species. Thus, it confirms that the reduction reaction can reach completion

with pure W being the product. The Faradaic efficiency of the process was

calculated by dividing the theoretical charge required by the charge passed.

The charge passed was calculated by adding the actual charge passed to an

assumed charge that would be passed given that the chronoamperometry

was allowed to reach zero potential. Experiments concluded that the current

efficiency for reducing WO3 to W in LiCl-KCl using the fluidised cathode

process is ~ 82%. The current efficiency was also calculated by subtracting

the background current as estimated from the pre-electrolysis, this was 4.3

mA, as presented in Figure 5.12(b). This reduces the errors from the

estimated extended current, and gives a new current efficiency of 100%.

Thus the current efficiency is 82-100%

When the final product was retrieved, a sample was placed in ethanol to

dissolve the salt, under an argon atmosphere. Dissolving the salt in water


was not appropriate in this case, as W, unlike Ti, reacts more readily with

water (and air) forming thin films of oxides, which was not desired for

analysis. After leaving the sample in ethanol for 24 hr, the ethanol

containing salt was filtered out using a small vacuum filtration unit. The

sample (powder) was then placed in ethanol again, and the procedure was

repeated twice. Finally, the product was dried in a vacuum oven at room

temperature. Despite this rigorous cleaning process, the XRD spectrum in

Fig. 12 confirms that KCl is still present in the sample. A solution to this

could be to employ a vacuum distillation [172, 173] unit at a high

temperature to remove the salt whilst still molten. The nature of the

fluidised cathode would complement such a filtration technique.




+, set

voltage: -2.135 V.


Figure 5.13 – X-ray diffraction spectrum (Mo Kα) of sample of product after complete

reduction, showing W (52344-ICSD [174]), and KCl (165593-ICSD [127]).

5.3.2 Effects of metal oxide – salt ratio

Constant potential experiments were carried out on different loadings of WO3

powder in LiCl-KCl salt eutectic. The voltage was again set to -2.14 V, Figure

5.14. Three chronoamperometry measurements were taken. Masses of WO3 (0.5,

40 and 60 g) were initially mixed with 150 g of LiCl-KCl in the main cell crucible,

with 10 g in the anode compartment, giving weight percentages of 0.312, 20.00 and

27.27 wt%, for the three different experiments. 0.5 g was chosen, as this was the

lowest limit at which some product could still be retrieved from the current

collector. 60 g was chosen, as this was the highest limit, given the ratio to 150 g of

salt, where the fluidisation process was still easily initiated. Although masses of up

to 100 g of WO3 could still be fluidised, they proved difficult for fluidisation

initiation and maintaining via gas bubbling.

The chronoamperograms show the same characteristic of an initial increase in

current. However, they all stabilise towards different currents, with the lowest

weight percentage achieving a lower current, than the higher weight percentage


experiments. Spalling-off is also witnessed, with associated sudden drops in

currents passed. The effect of metal oxide concentration in the melt on the

electrochemistry is attributed to the fact that statistically there are more frequent

interactions of particles with the electrode for a system with higher loading, thus

giving a higher total current. The difference in change in current with time is more

intriguing. The high oxide concentration leads to a monotonic increase in current

with time, while the lower concentrations show more signs of product spalling,

evident through the abrupt decrease in current. This would suggest that the product

is more stable in the high concentration case, allowing growth to continue and the

electrode to grow. At higher loadings the bulk density of the materials in the

reaction crucible is higher, which would hinder the current collector from vibrating

heavily, due to mechanical agitation via bubbling and collisions, which also

contributes to the spalling effect.

Figure 5.14 - Chronoamperograms of WO3 fluidised cathode in LiCl-KCl eutectic at

450 ˚C, argon flow rate: 800 cm3 min


+, set voltage: -2.14

V, for different weight percentages of WO3 in the melt, as indicated.


Figure 5.15 - Count of charge per spike relative to a normalised line of best fit, at

different WO3 – LiCl-KCl eutectic ratios. Where, (a) is at 0.312 wt% WO3, (b) 20.00

wt%, and (c) 27.27 wt%.

Figure 5.15 shows the count of charge per spike relative to a normalised line of

best fit for the chronoamperograms in Figure 5.14. 2500 points for each WO3

concentration were used to produce the data. It is very clear that with higher

concentrations the spike amplitude is much smaller. The increasing noise with the

decreasing oxide particle fraction is also attributed to the greater particle

homogeneity in the melt leading to a steadier current response. Thus, the noise

feature in the fluidised cathode process is attributed to the velocity (how long a

particle stays on the surface) with which the particles collide with the current

collector, rather than the number of particles impinging at any given point,

although each spike corresponds to more than one particle colliding.

5.3.3 Effects of fluidisation rate

Different argon flow rates were applied to the fluidised cathode process whilst

conducting a constant voltage experiment. Figure 5.16 shows the

chronoamperogram of the fluidised cathode under different agitation conditions.

The reducing potential was set to -2.14 V and the fluidisation rate was altered by

changing the argon flow rates from 200 cm3 min

-1 to a maximum of 1800 cm

3 min

-

1, and back to 200 cm

3 min

-1. It was increased by 200 cm

3 min

-1 every 5 minutes.

As time passed, the current increased, at each argon flow rate; again this is due to

an increase in the surface area of the current collector. It is evident that increasing


the agitation rate causes the current to increase more rapidly, and makes the

collision-reaction spikes (noise) greater. This indicates that a higher fluidisation

rate increases the likelihood of an encounter of an oxide particle with the electrode,

and will also increase the kinetic energy of particle collision with the electrode.

These effects are particularly acute above a threshold agitation level induced by an

argon flow rate of 1600 cm3

min-1

. In fluidised bed chemical engineering, there are

two main types of fluidisation regimes, depending on many parameters, but mainly

the superficial velocity of the fluid, the fluidisation regime can change from a

‘particulate’ to an ‘aggregative’ regime. This could be the explanation for such an

obvious change in the chronoamperommetry measurements. Further studies should

be conducted on the fluidisation regimes and their parameters, identification and

outcomes, to help understand the complexity of this system with three phases of

flow (including changes in density and other parameters, when electrochemical

alterations are applied).

Figure 5.17 shows the count of charge per spike relative to a line of best fit for the

chronoamperogram in Figure 5.16. Three hundred points for each argon flow rate

were used to plot the data. Here, one can clearly see that in the case of lower flow

rates the spikes are much smaller and lie toward the lower end of the charge per

spike spectrum. The opposite is true for higher agitation rates. This complements

the theory that the noise feature in the fluidised cathode process is a result of higher

impact velocity/kinetic energy. Thus, the effects of altering the fluidisation rates

are in agreement with metal oxide loading effects. Higher frequencies of collision-

reactions are responsible for electrode growth and current increase; and higher

impact velocities are responsible for higher noise, and ultimately spalling effects.



450 ˚C, 40 g WO3, reference electrode: Ag/Ag+, set voltage: -2.14 V, showing the effect

of different argon flow rates bubbled through the melt.

Figure 5.17 - Count of charge per spike relative to line of best fit, at different argon

flow rates. Where, (a) is at 200 cm3 min

-1, (b) 400 cm

3 min

-1, (c) 600 cm

3 min

-1, (d) 800

cm3 min

-1, (e) 1000 cm

3 min

-1, (f) 1200 cm

3 min

-1, (g) 1400 cm

3 min

-1, (h) 1600 cm

3 min

-

1, and (i) 1800 cm

3 min

-1.


5.3.4 Particle coulometric analysis

Particle coulometric analysis [175-178] was carried out on a section of 200 spikes

from the chronoamperometry in Figure 5.18 to characterise the particle-electrode

interaction dynamics. This section is presented in Figure 5.18 (b). Figure 5.19 (a)

shows the relative frequency of spike durations. The modal spike duration time is

0.4 s with a standard deviation of 0.2. Using Equation 5.10, the charge per spike

was calculated. This is presented in Figure 5.19 (b). The modal charge per spike is

-0.2 C with a standard deviation of 0.1.

𝑄 = ∫ 𝐼𝑑𝑡 = 𝑒𝑧𝑁 5.10

Where I is the current, t the spike duration, e the electronic charge = 1.602 10-19

C, z = 6 (the number of transferred electrons per reduced formula unit of WO3),

and N is the number of reduced WO3 units.

Assuming each spike on the chronoamperogram is only associated with one

particle impacting on the current collector, and that the particles are spherical in

shape, and fully reduced, Equation 5.11 can be used to estimate the size of the WO3

particles colliding with the working electrode, via cathodic coulometry.

𝑟 = √3𝑀𝑄

4𝜋𝑒𝑁𝐴𝑧𝜌

3 5.11

Where r is the radius of the WO3 particles, M = 231.84 g mol-1

(the molar mass of

WO3), NA = 6.022 1023

mol-1

(the Avogadro constant), = 7.16 g cm-3

(the

density of WO3).


Figure 5.18 - (a) Chronoamperogram of WO3 fluidised cathode in LiCl-KCl eutectic at

450 ˚C, 40 g WO3, reference electrode: Ag/Ag+, set voltage: -2.14 V, (b) zoom-in of a

section containing 200 spikes used for coulometric analysis.

Figure 5.19 - (a) Relative frequency of spike durations in Figure 5.18 (b) from

coulometric analysis. (b) Count of charge per spike from coulometric analysis.


Figure 5.20 - (a) Count of radii sizes of WO3 reactant particles from particle

coulommetry analysis. (b) SEM image of a large WO3 particle.

Figure 5.20 (a) shows the count of radii sizes as estimated from particle

coulometry. The mean radius size of WO3 is 154.5 μm with a standard deviation of

17. An SEM image of a large WO3 particle is shown in Figure 5.20 (b).

Particle size distribution analysis, Figure 5.1, showed that the actual mean particle

radius size is 15.41 μm. This discrepancy in results is expected because in the

literature, particle coulometry was used to predict the size of nano particles

impending on electrodes, whereas in the fluidised cathode system, where diffusion

of particles (in the micro scale) is enhanced via agitation of the melt, more than one

particle would be expected to collide with the current collector at any given stretch

of 0.2 s. Nonetheless, particle coulometry still provides a valuable understanding of

the system, as it concludes that on average, given the conditions applied in the

chronoampergram in Figure 5.18, a number of particles that amount to the size of

one spherical particle of 154.5 μm would impenge on the current collector at any

time band of 0.2 s and be fully reduced.


5.3.5 Electrochemical deposition model

An electrochemical deposition model for the fluidised cathode process was

developed in order to estimate the rate of growth of the deposit on the working

electrode, and the porosity characteristic of the deposit.

Assuming that the current density is constant across the entirety of the electrode at

different extents of reaction, the current, i, is related to the molar accumulation

over a given time, N(t), by Equation 5.12.

𝑖 = 𝑛𝐹𝑑

𝑑𝑡𝑁(𝑡) 5.12

Where, n is the number of moles and F is the Faraday constant.

Figure 5.21 – Schematic showing parameters of the current collector and the deposit

used in the electrochemical deposition model.

The volume deposited, V(t), is represented in Equation 5.13, and in terms of radii in

Equation 5.14, assuming the deposit is cylindrical in shape, perpendicular to the


current collector surface, Figure 5.21, which is in fact the case, as shown in Figure

5.10.

𝑉(𝑡) = 𝑁(𝑡)𝑀

𝜌 5.13

𝑉(𝑡) = 𝜋[𝑟(𝑡)2 − 𝑟02]𝐿 5.14

Where, M is the molar mass, is the density, 𝑟0 is the initial radius of the

electrode, r(t) is the radius after time, t, and L is the active length of the electrode.

Combining Equations 5.12, 5.13 and 5.14, Equations 5.15 and 5.16 can be derived.

𝑑

𝑑𝑡𝑟(𝑡)2 =

𝑖𝑀

𝜋𝐿𝑛𝐹𝜌= 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡𝑘 5.15

𝑟(𝑡) = √𝑘𝑡 + 𝑟02 5.16

Figure 5.23 is a plot of the electrochemical deposition model, with data (current)

acquired from the chronoamperommetry in Figure 5.22. It shows the evolution of

the working electrode’s radius size as time passes. With a starting radius of 0.75

mm, the model depicts that the final radius size after the electrolysis process is 1.90

mm. the size of this particular electrode was measured after experimentation with

no observed spalling off of the product, however, and was ~ 5.00 mm. This can be

attributed to a few reasons, such as the deposition morphology. Thus, the model as

it stands is not conclusive. Hence a more comprehensive deposition model should

be developed.





+, set

voltage: -2.14 V.

Figure 5.23 – Deposit growth on current collector over time, predicted via

electrochemical deposition model.


5.4 Conclusions

The electrochemical reduction of WO3 to W metal has been assessed, and it likely

to occur following the reactions WO3 → WO2 → W, Equations 5.1 and 5.2. A full

reduction using the fluidised cathode process, with complete conversion of the

product to W has been achieved, via applying a constant potential of -2.14 V. The

Faradaic current efficiency of the process has been established, and found to be ~

82%. The reduction process is split into two sections; the first where rapid

reduction of WO3 occurs, the second where a slower reduction of the remaining

oxides in the product are removed. The deposited material on the current collector

is in the form of homogeneously distributed particles.

Increasing the metal oxide – salt ratio results in increasing current (faster deposit

growth on the current collector), and decreasing collision-reaction noise, and in

less likelihood for product to spall off. Increasing the fluidisation rate of the

process results in an increase in the rate of deposit growth, as well as a greater

collision-reaction noise. The two main observations to take from this are:

a. The deposit growth, and ultimately the current increase, is dependent on

the frequency of particle – current collector collisions and reactions.

b. The collision-reaction noise (spikes) is dependent on the velocity at which

the particles collide with the current collector. Spalling of the product is

also highly affected by this.

Thus, depending on the desired means of recovery of the product, i.e. a continuous

flow retrieving or batch via removal of electrodes, these conditions can be altered

to suit.

Particle coulometric analysis shed some insight onto the process in terms of total

volume of particles impending on the current collector at any given time. An

electrochemical deposition model was also developed to estimate the porosity of

the deposit and the rate of its growth over time.


The fluidised cathode is a robust, three-phase, high efficiency process. It has been

studied here for the electrochemical reduction of tungsten oxide, however, it is

likely applicable for other refractory metals (such as titanium), and in the nuclear

industry for pyroprocessing purposes of spent nuclear materials.

In the following Chapter, the electrochemical reduction of UO2 to U metal is

investigated, the reaction pathways determined, and the fluidised cathode process is

used for the reduction process.

6. The Electrochemical Reduction of Uranium Oxide 108

6. The Electrochemical Reduction of Uranium Oxide

The electrochemical reduction of UO2 to U metal using a fluidised cathode process

was investigated. Voltammetry studies were conducted, and alongside a

predominance diagram that was constructed, the reaction path-way was studied,

using both, a fluidised cathode process and a packed metallic cavity electrode

precursor set-up. The route that the reduction process follows depends on pO2-

and

potential, which is highly influenced by the type of metal oxide precursor used.

The main reduction potential using the fluidised cathode appeared to be -2.2 V (vs.

Ag/Ag+ reference electrode). A Faradaic current efficiency for the process was

established, and found to be ~ 92%. It is proposed that the reduction process is split

into three stages; the first where a seeding process takes place at a low potential to

allow for the reduced uranium particles to be deposited onto the tungsten current

collector; the second where rapid reduction of UO2 particles occurs with a growth

in electrode size, accompanied by an increase in current total being passed; the

third where a slower reduction of the remaining oxides in the product takes place.

6.1 Introduction

High temperature molten salt reprocessing technology (pyroprocessing) offers a

range of advantages when compared to aqueous reprocessing techniques. This is

due to the fact that the technology is inherently ‘safe’ as it is resistant to

proliferation and does not provide an environment for criticality accidents to occur;

it also utilises compact easily accessible facilities. Molten salts pyroprocessing is

described in more detail in Chapter 2.

There is a clear motivation for implementing Generation IV nuclear reactors, which

require metal as start-up fuel and follow a fully integrated and safe reactor and

reprocessing design arrangements; hence, advances in the technology are made. A

conceptual flowsheet for the pyrochemical treatment of used LWR fuel, as

described by ANL [179], is presented in Figure 6.1. The spent nuclear fuel, in the


form of oxide pellets, is chopped and then passed on to an electrolytic reduction

step. The uranium is in the form of UO2; hence, the reduction of UO2 to U metal is

studied in this Chapter. The reduced species are then processed in an electrorefiner,

also described in Chapter 2, where U product and other transuranic species are

recovered for enriching and recycling into new fuel. The flowsheet comprises some

recycling streams and a salt distillation step as well.

Previous studies on the electrochemical reduction of spent nuclear fuel oxides and

uranium oxides are reported in detail in Chapter 2. In this Chapter, the

electrochemical reduction of UO2 to U metal in LiCl-KCl molten salt eutectic is

investigated, using both metallic cavity electrodes (MCE’s) and the fluidised

cathode process. The reduction pathways and the current efficiency are also

reported.

Figure 6.1 – Conceptual flowsheet for the treatment of used LWR fuel [179].


6.2 Experimental

6.2.1 Apparatus

A schematic of the electrolytic cell used is illustrated in Figure 3.13 (b). The

cathode consists of UO2 particles that are suspended in the molten salt (LiCl-KCl)

eutectic, and a pure tungsten rod current collector. The anode is a graphite rod

separated in its own compartment to avoid reoxidation of the reduced uranium

particles. The melt is agitated via a flow of argon. A reference electrode (Ag/Ag+)

was used, and the temperature was monitored via a thermocouple that is immersed

in the melt. All electrochemical tests were performed using a potentiostat

(IviumStat, Ivium Technologies, NL).

The same set-up was used to carry out the metallic cavity electrode (MCE)

experiments, Chapter 3. However, no UO2 particles were employed, the counter

electrode was not separated inside a compartment, no argon was bubbled through

the melt, and the current collector was replaced with an MCE, Figure 3.13 (a). The

cavities in the MCE were filled with UO2 powder. The same powder was used for

the fluidised cathode experiments.

6.2.2 Chemicals

All preparation steps were carried out under a sealed argon atmosphere. Anhydrous

lithium chloride (ACS reagent, ≥99.0% purity, Sigma-Aldrich) and potassium

chloride (≥99.5% purity, Sigma-Aldrich) were used for the electrolyte. The salt

was dried in a vacuum oven at 200 ˚C for 24 h, then 150 g of 59-41 mol% LiCl-

KCl were mixed with UO2 (as received from the Centre for Radiochemistry

Research, School of Chemistry, University of Manchester). Figure 6.2 shows the

X-ray diffraction pattern of the as-received UO2 powder. 15 g of LiCl-KCl was

placed inside the anode compartment. The counter electrode was a high density

graphite rod, 3.05 mm in diameter (99.9995% metal basis, Alfa Aesar). The

working electrode (current collector) was a tungsten rod, 1.5 mm in diameter

(99.95% metal basis, Alfa Aesar). A glass sheath around the shaft of the tungsten

rod insured that a constant surface area of electrode is exposed to the electrolyte,


even when being agitated by the Ar stream. Argon (99.998% purity, BOC) was

bubbled through the melt via a ceramic tube (5 mm internal diameter, Alsint).

For the MCE experiments, the electrolyte was composed of 150 g of LiCl-KCl,

prepared in the same way as for the fluidised cathode setup, but no UO2 particles

were agitated in the melt. The cavities in the working electrode were filled with

UO2 powder by ‘finger pressing’ using two glass slides.

Figure 6.2 - X-ray diffraction spectrum (Mo Kα) of as-received UO2 powder sample,

showing UO2 (35204-ICSD [180]).

6.2.3 Procedure

Experiments were carried out under a dry argon atmosphere at a melt temperature

of 450 ˚C (unless indicated otherwise). Argon was bubbled into the melt, which

resulted in a homogeneous distribution of particles as assessed by visual inspection.

4 cm (3.84 cm2) of the current collector was immersed in the melt during the

fluidised cathode measurements, and all the holes in the MCE, making sure that the


Mo wire and W rod stayed outside the melt. Absolute currents are reported due to

the fact that the electrode surface area changes during experiments.

6.3 Results and discussion

A predominance diagram, Chapter 4, was constructed for the Li-K-U-O-Cl system,

relating the potential E vs. standard chlorine electrode (S.Cl.E) to the negative

logarithm of O2-

ion activity, pO2-

. All the thermodynamic data used for the

production of the diagram is presented in Table 4.2, and the derived interface

equations are presented in Appendix B. A predominance diagram for uranium

species in LiCl-KCl has been published [181]; however, it did not consider all the

stable species that can form, and the thermodynamic data has been revised and

updated since.

The predominance diagram in Figure 4.9 shows the different regions of stability for

different compounds and oxidation states of uranium. Thermodynamically, one can

deduce that the concentration of O2-

ions in the eutectic melt greatly affects the

reduction process of UO2 to U metal. Starting with UO2, two reduction reactions

take place to produce U metal, according to the predominance diagram at 500 °C.

These are presented in Equations 6.1 and 6.2. Equations 6.3 and 6.4 are used to

calculate the potential, E, needed for each reaction to take place, at different values

of O2-

ion activity.

𝑈𝑂2 + 2𝑒− ↔ 𝑈𝑂 + 𝑂2− 6.1

𝑈𝑂 + 2𝑒− ↔ 𝑈 + 𝑂2− 6.2

𝐸6.1 =−∆𝐺6.1

0


2𝐹𝑝𝑂2− 6.3

𝐸6.2 =−∆𝐺6.2

0


2𝐹𝑝𝑂2− 6.4

The pO2-

ranges and potential bands (vs. S.Cl.E and Ag/Ag+ references electrode)

are presented in

Table 6.1, for the reactions described in Equations 6.1 and 6.2.



and potentials required for

Equations 6.1 and 6.2 to take place.

Reaction pO2- E (V vs. S.Cl.E) E (V vs. Ag/Ag+)

6.1 6.200 – 21.650 -3.558 ̶ -2.374 -2.422 – -1.238

6.2 6.500 – 21.750 -3.562 ̶ -2.394 -2.426 – -1.258

Cyclic voltammetry measurements were performed on an MCE molybdenum

electrode packed with UO2 powder; this is presented in Figure 6.3. It was scanned

from 0 V to -2.5 V, and back to 0 V (vs. Ag/Ag+ reference electrode). The coupled

redox potentials at 1, 2, 1’ and 2’ represent the reduction of a thin film of oxide on

the MCE molybdenum strip and its reoxidation. The redox pair A and A’ are

attributed to the reduction of UO2 to U metal, and its reoxidation to UOx (as it was

not confirmed what the oxidised species was via analysis), as shown in the overall

reaction described in Equation 6.5. This is in line with previous results [182].

𝑈𝑂2 + 4𝑒− ↔ 𝑈 + 2𝑂2− 6.5

Figure 6.4 shows cyclic voltammograms conducted on an MCE molybdenum

electrode packed with UO2 powder. These were scanned from 0 V to -2.5 V and

back to 0 V, at a variety of scan rates, as indicated in the diagram. Again, the redox

couple A and A’ are attributed to the reduction of UO2 to U metal, and its

reoxidation to UOx, as described in Equation 6.5. The reduction potential appears

to be very close to the salt’s decomposition potential; hence, indicating a high O2-

ion activity. As expected, at higher scan rates the reaction peaks are more

predominant.


Figure 6.3 - Cyclic voltammogram of UO2 in molybdenum metallic cavity electrode in

LiCl-KCl eutectic at 450 ˚C, scan rate: 20 mV s-1

, reference electrode: Ag/Ag+.

Figure 6.4 - Cyclic voltammograms of UO2 in molybdenum metallic cavity electrode,

at different scan rates, in LiCl-KCl eutectic at 450 ˚C, reference electrode: Ag/Ag+ (A

newly packed MCE was used for each scan).


Figure 6.5 – Cyclic voltammetry of UO2 fluidised cathode in LiCl-KCl eutectic at 450

°C, 5 g UO2, argon flow rate: 600 cm3 min


+.

Voltammetry measurements were performed on a fluidised cathode set-up, Figure

6.5. It was scanned from 0 V to -2.4 V, and back to 0 V (vs. Ag/Ag+ reference

electrode). The combination of the peaks at A and A’ represent the overall

reduction of UO2 to U metal as described by Equation 6.5. The peak at A’’

represents the reoxidation of U to UOx.

When comparing the voltammograms in Figure 6.5 (fluidised cathode) and Figure

6.3 (MCE), one can see that both peaks A and A’ appear earlier in the negative

direction, at ~ -1.7 V and ~ -2.2 V, in Figure 6.5, when compared to peak A in

Figure 6.3, which appears at ~ -2.4 V. This is due to the dynamic nature of the

fluidised cathode set-up. In the fluidise cathode process, a 3PI is immediately

initiated at the collision point of an oxide particle with the current collector, and a

reaction extends from this point. When using an MCE, the 3PI is defined at the

circumference of the packed metal oxide, where it meets the metal strip current

collector and the salt. This 3PI decreases as the reaction proceeds. Thus, in an

MCE set-up more driving force is required than when using a fluidised cathode

process, a higher overpotential. It is also likely that when using MCE’s that the

local O2-

ion concentration is higher.


Looking at the voltammogram in Figure 6.5 one can deduce that the reduction

starts quite early on at ~ -1.7 V and then concludes at ~ -2.2 V. This suggests that

using the fluidised cathode process both reactions, Equation 6.1 and 6.2, take place.

Thus, the reduction follows two reactions with 2-electron transfer step processes,

rather than the usual one 4-electron transfer step reaction (which could also be two

reactions with 2-electron transfer step processes that take place rapidly after one

another and thus appearing as one). Previous studies [183, 184] suggest that UO

can exist in equilibrium in the presence of other uranium oxide states. Using the

voltammetry information from Figure 6.3 and Figure 6.5, the reaction pathway for

the reduction of UO2 to U has been plotted on the predominance diagram in Figure

6.6, showing both the potential (vs. S.Cl.E) and the O2-

ion activity, as the reaction

evolves. One can see that the activity of O2-

using the fluidised cathode process is

generally lower. Also, it varies as the reaction proceeds, due to the fact that the

reaction lasts for a longer time.

Figure 6.6 - Predominance diagram for the Li-K-U-O-Cl system at 500 °C, showing

the reaction pathway for the reduction of UO2 to U in LiCl-KCl using a fluidised

cathode process and a metallic cavity electrode (MCE).


A constant potential of -2.2 V (vs. Ag/Ag+ reference electrode), as identified as the

reducing potential, was applied to the fluidised cathode system, Figure 6.7, 5 g of

UO2 powder was fluidised. As time passed, the current increased due to an increase

in the electrode surface area and the deposit growth of U on the current collector

surface. This growth is visible through the glass electrolytic cell set-up. At about

90,000 s, there is a rapid reduction in current associated with spalling off of the

deposit from the electrode. New growth is then associated with the increase in the

current, a similar characteristic of the process as seen for the reduction of WO3

[70].

When comparing the chronoamperogram to that for the reduction of tungsten

oxide, Figure 5.6, one can see that it takes a longer time for the electrode to start

growing, observed as an increase in the current. It is possible that a ‘seeding’ effect

whereby uranium metal takes some time to be deposited on the current collector.

This is possibly due to the fact that the current collector is made from a different

metal from that in the melt, compared with the all-tungsten system which did not

have this induction period.

Due to the difference in densities, when the electrolyte is left to solidify, different

layers of materials form, Figure 6.8 (c). Dark metallic uranium metal sinks to the

bottom, followed by a layer of black, still to be reduced, uranium oxide, and finally

a layer of LiCl-KCl eutectic (due to the similarity in appearance of U and UO2, it

was difficult to capture a clear edge between them). This could possibly provide a

simple means of separation in a technological system, as is the case with tungsten.

Figure 6.8 (a) shows an image of the current collector after the reduction has taken

place, and Figure 6.8 (b) shows an image of the cross-section of the reduced U

metal at the bottom of the reactor crucible.


Figure 6.7 - Chronoamperogram of UO2 fluidised cathode in LiCl-KCl eutectic at 450

°C, 5 g UO2, argon flow rate: 600 cm3 min

-1, set voltage: -2.2 V, reference electrode

Ag/Ag+.

Figure 6.8 – (a) Photograph of reduced uranium deposited on tungsten working

electrode, (b) photograph of the cross-section of the uranium product at the bottom of

the crucible, (c) photograph of the solidified product, showing two separate layers of

salt and uranium metal.


Figure 6.9 – SEM images of (a) the as-received UO2 particles, and (b) product U

obtained from the solidified melt as a separate layer to the LiCl-KCl.

There are two areas from where the final product could be collected, the bottom of

the cell crucible and on the surface of the working electrode. Figure 6.9 (a) shows

an SEM image of the as-received UO2 particles and (b) an SEM image of the U

metal product from the solidified melt. From the SEM images one can see that the

UO2 powder is very fine with particle sizes of ~1 μm, whereas the U product is in

the form of agglomerated particles forming a larger particle size fused with some

salt. Here, the use of vacuum distillation to separate the final product from the salt

would be beneficial.

6.3.1 Current efficiency

To establish the current efficiency of the process, a constant potential of -2.2 V (vs.

Ag/Ag+ reference electrode) was applied to the fluidised cathode set-up to reduce 4

g of UO2 to U metal, Figure 6.10. The chronoamperogram shows a similar trend to

previously published work on electrochemical reduction of metal oxides [58],

where the diagram can be segregated into two main stages; the first, where rapid

reduction reactions occur and the current increases significantly. In the second

stage, the current increases slightly again, then plateaus and slower reactions take

place to reduce the final oxides, until the current decreases indicative of the batch

reaction tending towards completion. Again, in Figure 6.10, one notices the


seeding time for the U particles to be deposited on the tungsten rod, as it takes time

for the electrode to start growing and the current to increase subsequently.

Assuming 100% current efficiency, it would require a charge of 5717.59 C to be

applied to fully reduce all 4 g of UO2. The final product retrieved from the

electrode’s surface was analysed via XRD, Figure 6.11. The spectrum shows the

spectra of KCl, and peaks associated with α-U. Most importantly, it shows that

there is no sign of any uranium oxide species, namely UO2 or UO. Thus, it

confirms that complete conversion is possible via the fluidised cathode

electrochemical process (in this case, unreacted oxide particles were presumably in

the solidified melt). Calculating an accurate Faradaic efficiency is challenging due

to various issues. To estimate the current efficiency, the curve toward the end of

the chronoamperometry, Figure 6.10, was extended to reach zero current,

estimating a constant gradient calculated from two points in the graph (7.52 × 105, -

6.10 × 10-3

and 8.64 × 105, -5.60 × 10

-3). The x-intercept was found to be 2.375 ×

106 s. The extended line was integrated to calculate the area, the hypothetical

charge passed, which was found to be 4566.24 C. This was added to the actual

charge passed, 1290 C, giving a total of 6230.10 C. The Faradaic efficiency of the

process was calculated by dividing the theoretical charge by the charge passed

(actual and hypothetical). Experiment concluded that the estimated current

efficiency for reducing UO2 to U in LiCl-KCl using the fluidised cathode process is

~ 92%. However, this is a very rough value, that would probably vary a lot.

When the final product was retrieved, a sample was placed in ethanol to dissolve

the salt, under an argon atmosphere, to avoid any reoxidation via reaction with air

or water, which was not desired for analysis. After leaving the sample in ethanol

for 24 hr, the ethanol containing salt was filtered out using a small vacuum

filtration unit. The sample (powder) was then placed in ethanol again, and the

procedure was repeated twice. Finally, the product was dried in a vacuum oven at

room temperature until dry. Despite this rigorous cleaning process, the XRD

spectrum in Figure 6.11 confirms that some KCl is still present in the sample. A

solution to this could be to employ vacuum distillation [172, 173] at high

temperature to remove the salt whilst still molten. The nature of the fluidised

cathode would complement such a filtration technique.



450 ˚C, 4 g UO2, argon flow rate: 600 cm3 min


+, set

voltage: -2.2 V.

Figure 6.11 - X-ray diffraction spectrum (Mo Kα) of sample of product after complete

reduction, showing peaks for α-U (43419-ICSD [185]), and KCl (165593-ICSD [127]).


6.4 Conclusions

The electrochemical reduction of UO2 to U metal has been assessed, and it is likely

to take place following two different reaction mechanisms; Equation 6.5, via a 4-

electron transfer step reaction; and Equations 6.1 and 6.2, via two 2-electron step

reactions. The route the reduction process follows depends on pO2-

and potential,

which is highly influenced by the type of metal oxide precursor used, MCE packed

electrode or a fluidised cathode.

The Faradaic current efficiency of the process has been estimated, via applying a

constant potential of -2.2 V, as identified by voltammetry measurements, and found

to be ~92%. The reduction process is split into three sections; the first where a

seeding process takes place at a low potential to allow for the reduced uranium

particles to be deposited onto the tungsten current collector; the second where rapid

reduction of UO2 particles occurs with a growth in electrode size accompanied by

an increase in current being passed; the third where a slower reduction of the

remaining oxides in the product occurs. The reduced product can be collected from

two areas; the deposit of the current collector and the bottom of the reactor

crucible.

The fluidised cathode is a robust, three-phase, high efficiency process. It has been

studied here for the electrochemical reduction of UO2; however, it might be

applicable for other spent fuel oxides (such as UO3 and PuO2), and in the

production of refractory metals, such as titanium. Further studies need to be

undertaken to validate this.

7. Conclusions and Future Work 123

7. Conclusions and Future Work

The aim of this research was to develop the understanding of spent nuclear

materials behaviour in molten salts, via thermodynamic and electrochemical

techniques, and to investigate electrochemical reduction processes from oxides to

metals, which can prove to be significant in future generation IV nuclear power

plants. New reactor designs are investigated as well, to improve the efficiency of

such processes. The main findings and suggested future work are summarised in

the Chapter.

7.1 Conclusions

7.1.1 Predominance Diagrams

Predominance phase diagrams for metal-molten salt systems are diagrams relating

the potential to the negative logarithm of the O2-

ion activity (E-pO2-

). They are a

useful tool for understanding the phase stability and electrochemistry of metal-

oxide systems in molten salts. They assist in predicting reaction pathways and

experimental results. They also give a good indication of whether a reaction is

feasible in a molten salt system.

Diagrams were produced for the range of spent nuclear materials (based on the

content of PWR MOX spent fuel). Thus, for U, Pu, Np, Am, Cm, Cs, Nd, Sm, Eu,

Gd, Mo, Tc, Ru, Rh, Ag and Cd species. The diagrams were constructed for two

salt systems; LiCl-KCl at 500 °C and NaCl-KCl at 750 °C. The two salt systems

were chosen as they are the two main systems used for pyroprocessing research by

ANL and RIAR, respectively. The temperatures were chosen within each salt’s

operating range.

All of the diagrams show regions of stability for the different metal species, their

oxides and chlorides at unit activity; nonetheless, the activity can be changed in

accordance to the equations that were derived.


The potential for selective direct reduction was also studied by superimposing the

diagrams of U and Pu, and Am and Cm onto each other, for both salt systems at the

two different temperatures. From the diagrams it was found that molten salt

pyroprocessing provides a valuable route for the reprocessing of nuclear materials

that is also resistant to proliferation, as it shows that it would be very challenging to

separate Pu from U. However, the approach is not seen to be a replacement for the

EXAm process, as the selective reduction and separation of Am from Cm would

also be challenging due to the similarity in reduction potential. The effect of

changing the operating temperature of the same salt system on the stability regions

was also examined. At higher temperatures, the stability zones are shifted to the

‘left’, at higher O2-

ion activities; however, at higher temperatures, the potential

window of the salt is smaller.

7.1.2 The electrochemical reduction of tungsten oxide

The electrochemical reduction of WO3 to W metal has been assessed, and is likely

to occur following the reaction mechanism WO3 → WO2 → W. A full reduction

using the fluidised cathode process was achieved, with complete conversion of the

product to W, as established via XRD analysis, by applying a constant potential of

-2.14 V. The Faradaic efficiency of the process was found to be ~ 82%. The

reduction process is split into two sections; the first where rapid reduction occurs,

and the second where a slower reduction of the remaining tungsten oxide particles

takes place. The product can be collected from the deposit on the current collector

and the bottom of the reaction crucible, and is in the form of homogenously

distributed particles.

Parameters, such as the fluidisation rate and the metal oxide – salt ratio, that affect

the fluidised cathode process were investigated. Experiments concluded that

increasing the fluidisation rate of the process results in an increase in the rate of

deposit growth, as well as a greater collision-reaction noise. Whilst increasing the

metal oxide – salt ratio results in increasing the rate at which the deposit growth

(and hence, the current), but in decreasing the collision-reaction noise, and thus,

decreasing the likelihood of product spall off. Therefore, the two main observations

from investigating these two parameters are: a) the deposit growth, and the current


increase, are dependent on the frequency of particle – current collector collisions

and reactions; b) the collision-reaction noise is dependent on the kinetic energy at

which the particles collide with the current collector (spalling off of the product is

highly related to this). Thus depending on the desired means of product recovery,

e.g. a contentious flow retrieving or batch via the removal of electrodes, these

conditions can be altered to suit.

Particle coulometric analyses has been carried out, which shed insight into the

process in terms of total volume of particles impending on the current collector and

being reduced at any given time. An electrochemical deposition model was also

developed to estimate the porosity of the deposited product on the current collector,

and the rate of its growth over time.

7.1.3 The electrochemical reduction of uranium oxide

The electrochemical reduction of UO2 to U metal was studied, and it is likely to

occur following two different reaction mechanisms; two 2-electron transfer

reactions from UO2 → UO → U, or one 4-electron reaction from UO2 → U. The

pathway for the reactions depends highly on the pO2-

and the potential, which is

greatly influenced by the type of uranium oxide precursor used; a fluidised cathode

or a packed MCE. Using the fluidised cathode provides a much larger potential

range at which the reduction reactions can occur.

The Faradaic efficiency of the reduction process using the fluidised cathode

method was estimated, via applying a constant potential of -2.2 V and extrapolating

the final decay to zero current, this was found to be ~ 92%. This process is inexact

and requires refinement of our ability to measure the converted product with

accuracy. An oxygen content analyser (LECO, St Joseph, MI, USA) would be very

beneficial to get accurate readings.

The reduction process is split into three stages; the first, contrary to reducing WO3

particles, involves an induction period, where low current is passed; this proposed

to be due to a ‘seeding’ step where a U layer is formed on the tungsten current

collector, onto which further reduction is more favourable. The second stage

involves rapid reduction of UO2 particles, as observed by the increase of current

(and growth of deposit on the current collector). The third stage shows a slower


reduction of the remaining uranium oxide particles as the reactant oxide is

exhausted. The reduced U product can, as is the case with tungsten, be collected

from the deposit on the current collector and the bottom of reactor crucible.

The fluidised cathode process is a robust three-dimensional, high efficiency

process. It also has other advantages, such as shortening the FFC process scheme,

with associated cost reductions, and the flexibility of deciding whether to apply it

for a contentious or a batch system. It has been studied for the electrochemical

reduction of WO3 and UO2 to their pure metal forms; however, it is likely

applicable for other spent oxide fuel materials (such as UO3 and PuO2), and in the

production of other refractory metals, such as titanium and tantalum.

7.2 Future work

7.2.1 Three dimensional microstructural analyses

Three dimensional microstructural analyses [186-191] of the deposit on the current

collector, using techniques such as X-ray computed tomography (X-ray CT) or

FIB/SEM, would be very beneficial, as it would provide parameters, such as the

porosity, for the electrochemical deposition model. This would be very useful for

defining different parameters for experiments, especially if a batch process was to

be employed, as this would help in defining the final electrode size with the

product deposit to be retrieved.

7.2.2 Single particle reduction analyses

Studying how a single particle of metal oxide would reduce and attach to the

current collector would be very interesting. Factors such as particle size and

geometry could be changed and investigated. Using techniques such as non-

destructive X-ray tomography and high speed filming, effects such as the change in

particle morphology via the reduction reaction and the ‘necking’ of the particle to

the current collector surface can be analysed.


Figure 0.1 – Experimental set-up for single particle reduction studies.

An experimental set-up for single particles studies has been designed, and is

presented in Figure 0.1. Here, a tungsten rod is covered in a Pyrex sheath, which

then widens, only exposing the top of the electrode, into a narrow well, where the

electrolyte and the particles to be reduced can be placed. The top of the well is

sealed with a suba-seal through which a graphite anode is pushed and immersed

into the fused salt. The exposed tungsten rod from the bottom and the graphite rod

at the top are connected to potentiostat leads, so that electrochemical experiments

can be carried out. The salt would be melted using an infrared laser, which based

on lab trial experiments is sufficient to reach desired temperatures.

7.2.3 ABBIS process

The ABBIS (Abdulaziz, Brett, Brown, Inman, Shearing) process is described in the

schematic shown in Figure 0.2. This is a concept that is an advancement of the

fluidised cathode process that aims to simplify it and make it more robust. Here,

the anode is a metal paste (potentially carbon or platinum) that is painted on the

outside of a crucible made of a suitable ion conductor. The idea is that the oxide

particles inside this crucible are reduced, whilst being agitated, and the O2-

ions

created via the reduction process would migrate through the salt melt and the ion

conductor and react with the anode on the outside, creating O2.


This process would benefit from simpler reactor design, easier retrieval of final

product, and possibly the reusability of the anode without degrading it. Foreseen

challenges include the use of an appropriate oxide ion conductor (that operates at a

relatively low temperature), molten salt electrolyte, temperature of process, pO2-

of

the melt, and metal oxide to be reduced. Nonetheless, there is a large amount of

literature to help in the design process, especially in choosing the right ion

conductor and metal-oxide system for the desired parameters [192, 193].

Figure 0.2 – Schematic for ABBIS process (with LiCl-KCl eutectic as an example).


7.2.4 Micro-electrode studies

It would be very beneficial to carry out studies on the fluidised cathode system

using micro-electrodes for molten salts [194, 195]. Using different salts and metal

oxides, these electrodes would be very beneficial in determining the types of

reactions that occur; reversible, quasi or non-reversible, when reducing a metal

oxide, as the reaction propagate downwards on a predominance diagram. This

would also help indicating whether activities of certain species change, thus

shifting equilibria, and ultimately result in making amendments to thermodynamic

predictions.

7.2.5 TRISO fuel pyroprocessing

Tristructural-isotropic (TRISO) fuel, the schematic of which is presented in Figure

0.3, is a micro particle fuel designed to be used in Generation VI very-high-

temperature reactors (VHTRs). It consists of an outer layer of pyrolytic carbon,

followed by a layer of silicon carbide, followed by an inner layer of pyrolytic

carbon, then a layer of porous carbon buffer that contains the fuel kernel, which is

typically composed of UOx, UC or UCO [196]. The TRISO particle is designed to

withstand the most catastrophic accidents without releasing the fuel.

Figure 0.3 – Illustrative cutaway drawing of a TRISO fuel particle [196].

Numerous studies on the manufacturing and integrity of TRISO particles have been

carried out; however, research in the processing of spent particles is deficient.


Previous studies of processing of TRISO particles were based on crushing the

graphite fuel blocks and manually separating the fuel particles from the graphite.

The outer carbon layer was then burned, followed by crushing of the silicon

carbide layer. The inner carbon layers were then oxidised as well. The remaining

ashes were then removed via leaching with nitric acid. The fuel was then removed

from the solution via conventional solvent extraction techniques [197-199]. This

technology involves many processing steps, and hence, it is costly. Some

laboratory scale pyroprocessing methods have been developed employing

electrorefining techniques [199]. The fluidised cathode process could prove very

efficient in the pyroprocessing of spent TRISO particles, where the different layers

could be stripped away at different potentials, ending with the fuel kernel particles,

which could then be reduced to their pure metal form and extracted to be reused for

metal fuelled Generation VI reactors.

7.2.6 Electrochemical reduction of UO3, PuO2 and mixed oxide fuels

Further studies on the reduction of spent nuclear materials can be carried out

integrating the fluidised cathode in the reprocessing scheme. Studying the

electrochemical reduction of UO3 and PuO2 is of particular interest, especially if an

integrated reprocessing scheme is to be adopted (pyroprocessing with solvent

extraction). With the help of predominance diagrams, the reaction pathways can be

determined, and the process optimised. Employing the fluidised cathode for

selective or full reduction of mixed actinides would be very interesting. Studying

the separation process to follow would also be interesting; whether by mixing and

settling in the reactor to see if different metals with different densities precipitate in

separate layers, or are split after if a contentious process is employed. This could

prove sufficient in the reprocessing cycle, eliminating steps such as centrifusion

with associated cost reductions.

7.2.7 Electrochemical reduction of ThO2

The use of thorium as a nuclear power fuel in new reactors has been suggested and

studied for some time [200, 201]. Thorium is a promising alternative to

conventional fuel types, as it contains a large amount of energy, is proliferation


resistant, and is more equally distributed worldwide. Bench scale experiments have

been carried out for the recovery of thorium fuel using solvent extraction

technology [202, 203]. One of the suggested Generation IV reactor designs is the

thorium molten salt reactor [204-206]. Thus, a natural choice for the recovery of

fuel from such a reactor would be by pyroprocessing. Here, the use of a fluidised

cathode might prove useful, and hence, investigating the technique would be

beneficial.

7.2.8 Electrochemical reduction of TiO2

Employing the fluidised cathode process for the reduction of refractory metal

oxides could prove beneficial, given its high Faradaic efficiency for the reduction

of WO3 and UO2. One such refractory metal to apply the process to is titanium. The

current efficiency for producing it via the FFC Cambridge process is quite low (10-

40% [51]). Hence, it would be very interesting to produce titanium using the

fluidised cathode process and comparing the efficiencies. Most of the studies on

the electrochemical reduction of TiO2 have been in CaCl2. It would be also worth

investigating the reduction in other molten salts such as LiCl-KCl eutectic, which

benefits from a lower operating temperature.

Preliminary studies on the electrochemical reduction of TiO2 to Ti metal in LiCl-

KCl have been carried out. A predominance diagram, Chapter 4, was constructed

for the Li-K-Ti-O-Cl system, Figure 0.4, relating the potential E vs. standard

chlorine electrode (S.Cl.E) to the negative logarithm of O2-

ions activity, pO2-

. The

derived interface equations are presented in Appendix D. A predominance diagram

for titanium species in CaCl2 has been published [131]; however, this is the first in

LiCl-KCl eutectic. Cyclic voltammetry measurements on thermally grown thin

TiO2 films were conducted as well, Figure 0.5. These show clear reduction and

reoxidation peaks (1, 2, 2’ and 1’, 3 and 3’ represent the reductive limit of the salt),

thus suggesting that the direct electrochemical reduction of titanium oxide in LiCl-

KCl is feasible. Further studies should be carried out to conclude the reduction

pathway, and finally, to utilise the fluidised cathode process and asses its

efficiency.


Figure 0.4 - Predominance diagram for the Li-K-Ti-O-Cl system at 500 °C.

Figure 0.5 - Cyclic voltammogram of TiO2 thin film cathode in LiCl-KCl eutectic at

450 °C, scan rate: 50 mV s-1

, reference electrode: Ag/Ag+.

Dissemination 133

Dissemination

Peer-reviewd publications

Abdulaziz, R., Brown, L. D., Inman, D., Simons, S. J., Shearing, P. R., Brett, D. J.

L. (2014). Effects of Process Conditions on the Fluidised Cathode Electrochemical

Reduction of Tungsten Oxide in Molten LiCl-KCl Eutectic. ECS Transactions,

64(4), 323-331.

Abdulaziz, R., Brown, L. D., Inman, D., Simons, S., Shearing, P. R., Brett, D. J. L.

(2014). A fluidised cathode process for the electrochemical reduction of tungsten

oxide in a molten LiCl-KCl eutectic. ECS Transactions, 58 (19), 65-74.


(2014). Novel fluidised cathode approach for the electrochemical reduction of

tungsten oxide in molten LiCl-KCl eutectic. Electrochemistry Communications,

41(0), 44-46.

Brown, L. D., Abdulaziz, R., Jervis, R., Bharath, V., Attwood, R., Reinhard, C.,

Connor, L.D., Simons, S.J.R., Inman, D., Brett, D.J.L. Shearing, P. R. (2015).

Following the electroreduction of uranium dioxide to uranium in LiCl-KCl eutectic

in situ using synchrotron radiation. Journal of Nuclear Materials. 464, 256-262.

Brown, L. D., Abdulaziz, R., Simons, S., Inman, D., Brett, D. J. L., Shearing, P. R.

(2013). Predominance diagrams of uranium and plutonum species in both lithium

chloride_potassium chloride eutectic and calcium chloride. Journal of Applied

Electrochemistry, 43(12), 1235-1241.

Dissemination 134

Conference presentations (oral)


(2015). Effects of Process Conditions on the Fluidised Cathode Electrochemical

Reduction of Tungsten Oxide in Molten LiCl-KCl Eutectic. Molten Salts

Discussion Group Summer Meeting, Cambridge.


(2014). Effects of Process Conditions on the Fluidised Cathode Electrochemical

Reduction of Tungsten Oxide in Molten LiCl-KCl Eutectic. ECS 226th Meeting,

Cancun.


(2013). A fluidised Cathode Process for the Direct Electrochemical Reduction of

Tungsten Oxide in Molten LiCl-KCl Eutectic. Molten Salts Discussion Group

Christmas Meeting, London.


(2013). Electrochemical Reduction of Tungsten Oxide in Molten LiCl-KCl Using a

Novel Fluidised Bed Electrode Approach. ECS 224th Meeting, San Francisco.

Conference presentations (poster)


(2014). Coulometric Analysis and the Effects of Fluidisation Rate on a Fluidised

Cathode for the electrochemical Reduction of Tungsten Oxide in a Molten LiCl-

KCl Eutectic. ChemEngDayUK 2014, Manchester.


(2013). Predominance Diagrams for U and Pu in LiCl-KCl. Electrochem 2013,

Southhampton.

Dissemination 135


(2013). Predominance Diagrams for U and Pu in LiCl-KCl. Molten Salts

Discussion Group Summer Meeting, Cambridge.

Appendices 136

Appendices

Appendix A – calculations for immersion heater

To calculate the heat input needed to increase the temperature of the thermostatic

salt bath Equations A.1 and A.2 are used.

𝑄 = 𝑚𝑐∆𝑇 +𝑚𝐿 A.1

𝑄 = 𝑚𝑐∆𝑇 A.2

Where, Q is heat input in J;

m is mass of thermostatic salt used = 3000 g;

c is specific heat capacity of the salt = 1.592 J g-1

˚C-1

;

ΔT is change in temperature of the salt = Tf – Ts = desired

temperature in ˚C - 20˚C (temperature of the salt at the start);

L is latent heat of the salt = 97 J g-1

.

Equation A.1 is used when the required final temperature of the molten salt mixture

is higher than 220 °C, and Equation A.2 is used when the desired temperature is

lower than 220 °C. Thus, Q is the heat output needed to be transferred from the

nichrome wire of the immersion heater to the salt eutectic. The nichrome wire is 4

m long and 0.46 mm in diameter. Hence, its resistance is 26.63 Ω. To allow

sufficient time for the wire and the salt bath to heat up, 1.5 hrs heating time is

assigned. Hence:

Appendices 137

𝑃 =𝑄

60×90𝑠𝑒𝑐𝑜𝑛𝑑𝑠 A.3

Where, P is power in J s-1

.

Equation A.4, rearranged as A.5, is then used to calculate the voltage needed.

𝑃 = 𝐼2𝑅 = (𝑉

𝑅)2𝑅 A.4

𝑉 = 𝑅√𝑃

𝑅 A.5

Where, I is current in A;

R is resistance in Ω.

Table 0.1, shows the calculated heat, power and voltage for a given final

temperature of the thermostatic salt bath. Heat loss to the environment is not

accounted for; however these values give a good indication for the range of voltage

needed to be supplied through the variac.

Table 0.1 - Voltage, power and heat needed to reach specified temperatures of the

thermostatic salt bath.

T (˚C) Q (J) P (J s-1) V (V)

600 3061000 567 123

500 2583000 478 113

400 2106000 390 102

300 1628000 302 90

200 860000 159 65

100 382000 71 44

Appendices 138

Appendix B – Electrode potential and pO2-

equations for predominance diagrams

of spent nuclear materials

Uranium (U) species

𝑈𝑂 + 2𝑒− ↔ 𝑈 + 𝑂2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

𝑈𝑂2 + 2𝑒− ↔ 𝑈𝑂 + 𝑂2− 𝐸 =

−∆𝐺𝑜


2𝐹𝑝𝑂2−

𝑈4𝑂9 + 2𝑒− ↔ 4𝑈𝑂2 + 𝑂

2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

4𝑈3𝑂8 + 10𝑒− ↔ 3𝑈4𝑂9 + 5𝑂

2− 𝐸 =−∆𝐺𝑜


10𝐹𝑝𝑂2−

3𝑈𝑂3 + 2𝑒− ↔ 𝑈3𝑂8 + 𝑂

2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

𝑈2𝐶𝑙5𝑂2 + 𝑒− ↔ 2𝑈𝐶𝑙2𝑂 + 𝐶𝑙

− 𝐸 =−∆𝐺𝑜

𝐹

2𝑈𝑂3 + 5𝐶𝑙− + 3𝑒− ↔ 𝑈2𝐶𝑙5𝑂2 + 4𝑂

2− 𝐸 =−∆𝐺𝑜


3𝐹𝑝𝑂2−

2𝑈3𝑂8 + 15𝐶𝑙− + 5𝑒− ↔ 3𝑈2𝐶𝑙5𝑂2 + 10𝑂

2− 𝐸 =−∆𝐺𝑜


5𝐹𝑝𝑂2−

𝑈𝑂2 + 2𝐶𝑙− ↔ 𝑈𝐶𝑙2𝑂 + 𝑂

2− 𝑝𝑂2− =∆𝐺𝑜

𝑅𝑇𝑙𝑛10

𝑈𝑂 + 2𝐶𝑙− − 2𝑒− ↔ 𝑈𝐶𝑙2𝑂 𝐸 =∆𝐺𝑜

2𝐹

𝑈𝐶𝑙2𝑂 + 4𝑒− ↔ 𝑈 +𝑂2− + 2𝐶𝑙− 𝐸 =

−∆𝐺𝑜


4𝐹𝑝𝑂2−

𝑈3+ + 3𝑒− ↔ 𝑈 𝐸 =−∆𝐺𝑜

3𝐹

𝑈4+ + 𝑒− ↔ 𝑈3+ 𝐸 =−∆𝐺𝑜

𝐹

𝑈𝑂 + 3𝐶𝑙− − 𝑒− ↔ 𝑈𝐶𝑙3 + 𝑂2− 𝐸 =

∆𝐺𝑜

𝐹−𝑅𝑇𝑙𝑛10

𝐹𝑝𝑂2−

𝑈𝑂2 + 3𝐶𝑙− + 𝑒− ↔ 𝑈𝐶𝑙3 + 2𝑂

2− 𝐸 =−∆𝐺𝑜

𝐹+2𝑅𝑇𝑙𝑛10

𝐹𝑝𝑂2−

𝑈𝐶𝑙2𝑂 + 𝐶𝑙− + 𝑒− ↔ 𝑈𝐶𝑙3 + 𝑂

2− 𝐸 =−∆𝐺𝑜

𝐹+𝑅𝑇𝑙𝑛10

𝐹𝑝𝑂2−

𝑈𝐶𝑙2𝑂 + 2𝐶𝑙− ↔ 𝑈𝐶𝑙4 + 𝑂

2− 𝑝𝑂2− =∆𝐺𝑜

𝑅𝑇𝑙𝑛10

𝑈2𝐶𝑙5𝑂2 + 3𝐶𝑙− + 𝑒− ↔ 2𝑈𝐶𝑙4 + 2𝑂

2− 𝐸 =−∆𝐺𝑜


𝐹𝑝𝑂2−

𝑈4𝑂9 + 8𝐶𝑙− + 2𝑒− ↔ 4𝑈𝐶𝑙2𝑂 + 5𝑂

2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

𝑈4𝑂9 + 10𝐶𝑙− ↔ 2𝑈2𝐶𝑙5𝑂2 + 5𝑂

2− 𝑝𝑂2− =∆𝐺𝑜

5𝑅𝑇𝑙𝑛10

Appendices 139

𝐿𝑖2𝑈𝑂4 + 2𝑒− ↔ 𝑈𝑂2 + 𝐿𝑖2𝑂 + 𝑂

2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

4𝐿𝑖2𝑈𝑂4 + 6𝑒− ↔ 𝑈4𝑂9 + 4𝐿𝑖2𝑂 + 3𝑂

2− 𝐸 =−∆𝐺𝑜


6𝐹𝑝𝑂2−

3𝐿𝑖2𝑈𝑂4 + 2𝑒− ↔ 𝑈3𝑂8 + 3𝐿𝑖2𝑂 + 𝑂

2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

𝐿𝑖2𝑈𝑂4 ↔ 𝑈𝑂3 + 𝐿𝑖2𝑂 𝑝𝑂2− =∆𝐺𝑜

𝑅𝑇𝑙𝑛10

Plutonium (Pu) species

𝑃𝑢2𝑂3 + 6𝑒− ↔ 2𝑃𝑢 + 3𝑂2− 𝐸 =

−∆𝐺𝑜


6𝐹𝑝𝑂2−

2𝑃𝑢𝑂2 + 2𝑒− ↔ 𝑃𝑢2𝑂3 + 𝑂

2− 𝐸 =−∆𝐺𝑜


2𝐹𝑝𝑂2−

𝑃𝑢3+ + 3𝑒− ↔ 𝑃𝑢 𝐸 =−∆𝐺𝑜

3𝐹

𝑃𝑢2𝑂3 + 2𝐶𝑙− ↔ 2𝑃𝑢𝐶𝑙𝑂 + 𝑂2− 𝑝𝑂2− =

∆𝐺𝑜

𝑅𝑇𝑙𝑛10

𝑃𝑢𝑂2 + 𝐶𝑙− + 𝑒− ↔ 𝑃𝑢𝐶𝑙𝑂 + 𝑂2− 𝐸 =

−∆𝐺𝑜


𝐹𝑝𝑂2−

𝑃𝑢𝐶𝑙𝑂 + 2𝐶𝑙− ↔ 𝑃𝑢𝐶𝑙3 + 𝑂2− 𝑝𝑂2− =

∆𝐺𝑜

𝑅𝑇𝑙𝑛10

𝑃𝑢𝐶𝑙𝑂 + 3𝑒− ↔ 𝑃𝑢 + 𝐶𝑙− +𝑂2− 𝐸 =−∆𝐺𝑜


3𝐹𝑝𝑂2−

𝑃𝑢𝑂2 + 3𝐶𝑙− + 𝑒− ↔ 𝑃𝑢𝐶𝑙3 + 2𝑂

2− 𝐸 =−∆𝐺𝑜


𝐹𝑝𝑂2−

Neptunium (Np) species

𝑁𝑝 + 2𝑂2− ↔ 𝑁𝑝𝑂2 + 4𝑒− 𝐸 =

−∆𝐺0


4𝐹𝑝𝑂2−

2𝑁𝑝𝑂2 + 𝑂2− ↔ 𝑁𝑝2𝑂5 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑁𝑝𝑂2 + 𝐿𝑖2𝑂 + 3𝑂2− ↔ 𝐿𝑖2𝑁𝑝𝑂6 + 6𝑒

− 𝐸 =−∆𝐺0


6𝐹𝑝𝑂2−

𝑁𝑝2𝑂5 + 2𝐿𝑖2𝑂 + 5𝑂2− ↔ 2𝐿𝑖2𝑁𝑝𝑂6 + 10𝑒

− 𝐸 =−∆𝐺0


10𝐹𝑝𝑂2−

𝑁𝑝 ↔ 𝑁𝑝3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝑁𝑝3+ ↔ 𝑁𝑝4+ + 𝑒− 𝐸 =−∆𝐺0

𝐹

𝑁𝑝𝐶𝑙3 + 2𝑂2− ↔ 𝑁𝑝𝑂2 + 3𝐶𝑙

− + 𝑒− 𝐸 =−∆𝐺0


𝐹𝑝𝑂2−

Appendices 140

𝑁𝑝𝐶𝑙4 + 2𝑂2− ↔ 𝑁𝑝𝑂2 + 4𝐶𝑙

− 𝑝𝑂2− =∆𝐺0

2𝑅𝑇𝑙𝑛10

Americium (Am) species

2𝐴𝑚 + 3𝑂2− ↔ 𝐴𝑚2𝑂3 + 6𝑒− 𝐸 =

−∆𝐺0


6𝐹𝑝𝑂2−

𝐴𝑚2𝑂3 + 𝑂2− ↔ 2𝐴𝑚𝑂2 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝐴𝑚 ↔ 𝐴𝑚3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝐴𝑚 + 𝑂2− + 𝐶𝑙− ↔ 𝐴𝑚𝐶𝑙𝑂 + 3𝑒− 𝐸 =−∆𝐺0


3𝐹𝑝𝑂2−

2𝐴𝑚𝐶𝑙𝑂 + 𝑂2− ↔ 𝐴𝑚2𝑂3 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝐴𝑚𝐶𝑙𝑂 + 𝑂2− ↔ 𝐴𝑚𝑂2 + 𝐶𝑙− + 𝑒− 𝐸 =

−∆𝐺0


𝐹𝑝𝑂2−

𝐴𝑚𝐶𝑙3 + 𝑂2− ↔ 𝐴𝑚𝐶𝑙𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

Curium (Cm) species

2𝐶𝑚 + 3𝑂2− ↔ 𝐶𝑚2𝑂3 + 6𝑒− 𝐸 =

−∆𝐺0


6𝐹𝑝𝑂2−

𝐶𝑚2𝑂3 +𝑂2− ↔ 2𝐶𝑚𝑂2 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝐶𝑚 ↔ 𝐶𝑚3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝐶𝑚 + 𝑂2− + 𝐶𝑙− ↔ 𝐶𝑚𝐶𝑙𝑂 + 3𝑒− 𝐸 =−∆𝐺0


3𝐹𝑝𝑂2−

2𝐶𝑚𝐶𝑙𝑂 + 𝑂2− ↔ 𝐶𝑚2𝑂3 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝐶𝑚𝐶𝑙𝑂 + 𝑂2− ↔ 𝐶𝑚𝑂2 + 𝐶𝑙− + 𝑒− 𝐸 =

−∆𝐺0


𝐹𝑝𝑂2−

𝐶𝑚𝐶𝑙3 +𝑂2− ↔ 𝐶𝑚𝐶𝑙𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

Caesium (Cs) species

2𝐶𝑠 + 𝑂2− ↔ 𝐶𝑠2𝑂 + 2𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

𝐶𝑠2𝑂 + 𝑂2− ↔ 𝐶𝑠2𝑂2 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

Appendices 141

𝐶𝑠2𝑂2 + 𝑂2− ↔ 𝐶𝑠2𝑂3 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝐶𝑠2𝑂3 + 𝑂2− ↔ 𝐶𝑠𝑂2 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

Neodymium (Nd) species

2𝑁𝑑 + 3𝑂2− ↔ 𝑁𝑑2𝑂3 + 6𝑒− 𝐸 =

−∆𝐺0


6𝐹𝑝𝑂2−

𝑁𝑑2𝑂3 + 𝑂2− ↔ 2𝑁𝑑𝑂2 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑁𝑑 + 𝑂2− + 𝐶𝑙− ↔ 𝑁𝑑𝐶𝑙𝑂 + 3𝑒− 𝐸 =−∆𝐺0


3𝐹𝑝𝑂2−

2𝑁𝑑𝐶𝑙𝑂 + 𝑂2− ↔𝑁𝑑2𝑂3 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝑁𝑑𝐶𝑙3 +𝑂2− ↔ 𝑁𝑑𝐶𝑙𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝑁𝑑 ↔ 𝑁𝑑3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

Samarium (Sm) species

2𝑆𝑚 + 3𝑂2− ↔ 𝑆𝑚2𝑂3 + 6𝑒− 𝐸 =

−∆𝐺0


6𝐹𝑝𝑂2−

𝑆𝑚 + 𝑂2− + 𝐶𝑙− ↔ 𝑆𝑚𝐶𝑙𝑂 + 3𝑒− 𝐸 =−∆𝐺0


3𝐹𝑝𝑂2−

2𝑆𝑚𝐶𝑙𝑂 + 𝑂2− ↔ 𝑆𝑚2𝑂3 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝑆𝑚𝐶𝑙3 + 𝑂2− ↔ 𝑆𝑚𝐶𝑙𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝑆𝑚 ↔ 𝑆𝑚3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

Europium (Eu) species

𝐸𝑢 + 𝑂2− ↔ 𝐸𝑢𝑂 + 2𝑒− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

2𝐸𝑢𝑂 + 𝑂2− ↔ 𝐸𝑢2𝑂3 + 2𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

𝐸𝑢 ↔ 𝐸𝑢3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝐸𝑢𝑂 + 𝐶𝑙− ↔ 𝐸𝑢𝐶𝑙𝑂 + 𝑒− 𝐸 =−∆𝐺0

𝐹

Appendices 142

𝐸𝑢𝐶𝑙3 +𝑂2− ↔ 𝐸𝑢𝐶𝑙𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

2𝐸𝑢𝐶𝑙𝑂 + 𝑂2− ↔ 𝐸𝑢2𝑂3 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝐸𝑢𝐶𝑙3 +𝑂2− ↔ 𝐸𝑢𝑂 + 3𝐶𝑙− − 𝑒− 𝐸 =

∆𝐺0


𝐹𝑝𝑂2−

Gadolinium (Gd) species

2𝐺𝑑 + 3𝑂2− ↔ 𝐺𝑑2𝑂3 + 6𝑒− 𝐸 =

−∆𝐺0


6𝐹𝑝𝑂2−

𝐺𝑑 + 𝑂2− + 𝐶𝑙− ↔ 𝐺𝑑𝐶𝑙𝑂 + 3𝑒− 𝐸 =−∆𝐺0


3𝐹𝑝𝑂2−

2𝐺𝑑𝐶𝑙𝑂 + 𝑂2− ↔ 𝐺𝑑2𝑂3 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝐺𝑑𝐶𝑙3 + 𝑂2− ↔ 𝐺𝑑𝐶𝑙𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝐺𝑑 ↔ 𝐺𝑑3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

Molybdenum (Mo) species

𝑀𝑜 + 2𝑂2− ↔ 𝑀𝑜𝑂2 + 4𝑒− 𝐸 =

−∆𝐺0


4𝐹𝑝𝑂2−

𝑀𝑜𝑂2 +𝑂2− ↔ 𝑀𝑂3 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑀𝑜𝑂2 + 𝐿𝑖2𝑂 + 𝑂2− ↔ 𝐿𝑖2𝑀𝑜𝑂4 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑀𝑜𝑂3 + 𝐿𝑖2𝑂 ↔ 𝐿𝑖2𝑀𝑜𝑂4 𝑝𝑂2− =∆𝐺0

𝑅𝑇𝑙𝑛10

𝑀𝑜 + 𝐿𝑖2𝑂 + 3𝑂2− ↔ 𝐿𝑖2𝑀𝑜𝑂4 + 6𝑒

− 𝐸 =−∆𝐺0


6𝐹𝑝𝑂2−

𝑀𝑜𝐶𝑙2𝑂 + 𝑂2− ↔ 𝑀𝑜𝑂2 + 2𝐶𝑙

− 𝑝𝑂2− =∆𝐺0

𝑅𝑇𝑙𝑛10

𝑀𝑜 ↔ 𝑀𝑜2+ + 2𝑒− 𝐸 =−∆𝐺0

2𝐹

𝑀𝑜2+ ↔ 𝑀𝑜5+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝑀𝑜𝐶𝑙2 + 2𝑂2− ↔ 𝑀𝑜𝑂2 + 2𝐶𝑙

− + 2𝑒− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−


− − 𝑒− 𝐸 =∆𝐺0

𝐹−2𝑅𝑇𝑙𝑛10

𝐹𝑝𝑂2−

𝑀𝑜𝐶𝑙2 + 𝑂2− ↔ 𝑀𝑜𝐶𝑙2𝑂 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

Appendices 143

𝑀𝑜𝐶𝑙5 + 𝑂2− ↔ 𝑀𝑜𝐶𝑙2𝑂 + 3𝐶𝑙

− − 𝑒− 𝐸 =∆𝐺0


𝐹𝑝𝑂2−


− + 𝑒− 𝐸 =−∆𝐺0


𝐹𝑝𝑂2−

Technetium (Tc) species

𝑇𝑐 + 2𝑂2− ↔ 𝑇𝑐𝑂2 + 4𝑒− 𝐸 =

−∆𝐺0


4𝐹𝑝𝑂2−

𝑇𝑐𝑂2 + 𝑂2− ↔ 𝑇𝑐𝑂3 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

2𝑇𝑐𝑂3 + 𝑂2− ↔ 𝑇𝑐2𝑂7 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑇𝑐 ↔ 𝑇𝑐3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝑇𝑐𝐶𝑙3 + 2𝑂2− ↔ 𝑇𝑐𝑂2 + 3𝐶𝑙

− + 𝑒− 𝐸 =−∆𝐺0


𝐹𝑝𝑂2−

Ruthenium (Ru) species

𝑅𝑢 + 2𝑂2− ↔ 𝑅𝑢𝑂2 + 4𝑒− 𝐸 =

−∆𝐺0


4𝐹𝑝𝑂2−

𝑅𝑢𝑂2 + 2𝑂2− ↔ 𝑅𝑢𝑂4 + 4𝑒

− 𝐸 =−∆𝐺0


4𝐹𝑝𝑂2−

𝑅𝑢 ↔ 𝑅𝑢3+ + 3𝑒− 𝐸 =−∆𝐺0

3𝐹

𝑅𝑢𝐶𝑙3 + 2𝑂2− ↔ 𝑅𝑢𝑂2 + 3𝐶𝑙

− + 𝑒− 𝐸 =−∆𝐺0


𝐹𝑝𝑂2−

Rhodium (Rh) species

2𝑅ℎ + 𝑂2− ↔ 𝑅ℎ2𝑂 + 2𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

𝑅ℎ2𝑂 + 𝑂2− ↔ 2𝑅ℎ𝑂 + 2𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

𝑅ℎ ↔ 𝑅ℎ+ + 𝑒− 𝐸 =−∆𝐺0

𝐹

𝑅ℎ+ ↔ 𝑅ℎ2+ + 𝑒− 𝐸 =−∆𝐺0

𝐹

𝑅ℎ2+ ↔ 𝑅ℎ3+ + 𝑒− 𝐸 =−∆𝐺0

𝐹

2𝑅ℎ𝐶𝑙 + 𝑂2− ↔ 𝑅ℎ2𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

Appendices 144

2𝑅ℎ𝐶𝑙3 + 𝑂2− ↔ 𝑅ℎ2𝑂 + 6𝐶𝑙

− − 4𝑒− 𝐸 =∆𝐺0

4𝐹−𝑅𝑇𝑙𝑛10

4𝐹𝑝𝑂2−

𝑅ℎ𝐶𝑙3 + 𝑂2− ↔ 𝑅ℎ𝑂 + 3𝐶𝑙− − 𝑒− 𝐸 =

∆𝐺0


𝐹𝑝𝑂2−

Cadmium (Cd) species

𝐶𝑑 + 𝑂2− ↔ 𝐶𝑑𝑂 + 2𝑒− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝐶𝑑 ↔ 𝐶𝑑2+ + 2𝑒− 𝐸 =−∆𝐺0

2𝐹

𝐶𝑑𝐶𝑙2 + 𝑂2− ↔ 𝐶𝑑𝑂 + 2𝐶𝑙− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

Appendices 145

Appendix C - Electrode potential and pO2-

equations for the Li-K-W-O-Cl system’s

predominance diagram

𝑊𝑂2 + 4𝑒− ↔𝑊 + 2𝑂2− 𝐸 =

−∆𝐺0


4𝐹𝑝𝑂2−

𝑊𝑂3 + 2𝑒− ↔𝑊𝑂2 + 𝑂

2− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑊2+ + 2𝑒− ↔𝑊 𝐸 =−∆𝐺0

2𝐹

𝑊4+ + 2𝑒− ↔𝑊2+ 𝐸 =−∆𝐺0

2𝐹

𝑊𝑂3 + 2𝐶𝑙− ↔𝑊𝑂2𝐶𝑙2 +𝑂

2− 𝑝𝑂2− =∆𝐺0

𝑅𝑇𝑙𝑛10

𝑊𝑂2 + 2𝐶𝑙− − 2𝑒− ↔𝑊𝑂2𝐶𝑙2 𝐸 =

∆𝐺0

2𝐹

𝑊𝑂2𝐶𝑙2 + 6𝑒− ↔𝑊 + 2𝑂2− + 2𝐶𝑙− 𝐸 =

−∆𝐺0


6𝐹𝑝𝑂2−

𝑊𝑂2𝐶𝑙2 + 2𝐶𝑙− + 2𝑒− ↔𝑊𝐶𝑙4 + 2𝑂

2− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑊𝑂2𝐶𝑙2 + 4𝑒− ↔𝑊𝐶𝑙2 + 2𝑂

2− 𝐸 =−∆𝐺0


4𝐹𝑝𝑂2−

𝑊𝑂2 + 2𝐶𝑙− + 2𝑒− ↔𝑊𝐶𝑙2 + 2𝑂

2− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

References 146

Appendix D - Electrode potential and pO2-

equations for the Li-K-Ti-O-Cl system’s

predominance diagram

3𝑇𝑖 + 2𝑂2− ↔ 𝑇𝑖3𝑂2 + 4𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

𝑇𝑖3𝑂2 + 𝑂2− ↔ 3𝑇𝑖𝑂 + 2𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

2𝑇𝑖𝑂 + 𝑂2− ↔ 𝑇𝑖2𝑂3 + 2𝑒− 𝐸 =

−∆𝐺0


2𝐹𝑝𝑂2−

3𝑇𝑖2𝑂3 + 𝑂2− ↔ 2𝑇𝑖3𝑂5 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝑇𝑖3𝑂5 + 𝑂2− ↔ 3𝑇𝑖𝑂2 + 2𝑒

− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝐿𝑖2𝑇𝑖𝑂3 ↔ 𝑇𝑖𝑂2 + 𝐿𝑖2𝑂 𝑝𝑂2− =∆𝐺0

𝑅𝑇𝑙𝑛10

3𝐿𝑖2𝑇𝑖𝑂3 + 2𝑒− ↔ 𝑇𝑖3𝑂5 + 3𝐿𝑖2𝑂 + 𝑂

2− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

2𝐿𝑖2𝑇𝑖𝑂3 + 2𝑒− ↔ 𝑇𝑖2𝑂3 + 2𝐿𝑖2𝑂 + 𝑂

2− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

𝐿𝑖2𝑇𝑖𝑂3 + 2𝑒− ↔ 𝑇𝑖𝑂 + 𝐿𝑖2𝑂 + 𝑂

2− 𝐸 =−∆𝐺0


2𝐹𝑝𝑂2−

3𝐿𝑖2𝑇𝑖𝑂3 + 6𝑒− ↔ 𝑇𝑖3𝑂2 + 3𝐿𝑖2𝑂 + 3𝑂

2− 𝐸 =−∆𝐺0


6𝐹𝑝𝑂2−

𝐿𝑖2𝑇𝑖𝑂3 + 4𝑒− ↔ 𝑇𝑖 + 𝐿𝑖2𝑂 + 2𝑂

2− 𝐸 =−∆𝐺0


4𝐹𝑝𝑂2−

𝑇𝑖𝑂2 + 𝐶𝑙− + 𝑒− ↔ 𝑇𝑖𝐶𝑙𝑂 + 𝑂2− 𝐸 =

−∆𝐺0


𝐹𝑝𝑂2−

𝑇𝑖3𝑂5 + 3𝐶𝑙− + 𝑒− ↔ 3𝑇𝑖𝐶𝑙𝑂 + 2𝑂2− 𝐸 =

−∆𝐺0


𝐹𝑝𝑂2−

𝑇𝑖2𝑂3 + 2𝐶𝑙− ↔ 2𝑇𝑖𝐶𝑙𝑂 + 𝑂2− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝑇𝑖𝐶𝑙𝑂 + 𝑒− ↔ 𝑇𝑖𝑂 + 𝐶𝑙− 𝐸 =−∆𝐺0

𝐹

𝑇𝑖𝐶𝑙𝑂 + 3𝐶𝑙− − 𝑒− ↔ 𝑇𝑖𝐶𝑙4 +𝑂2− 𝐸 =

∆𝐺0


𝐹𝑝𝑂2−

𝑇𝑖𝐶𝑙𝑂 + 2𝐶𝑙− ↔ 𝑇𝑖𝐶𝑙3 +𝑂2− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

𝑇𝑖𝐶𝑙𝑂 + 𝐶𝑙− + 𝑒− ↔ 𝑇𝑖𝐶𝑙2 + 𝑂2− 𝐸 =

−∆𝐺0


𝐹𝑝𝑂2−

𝑇𝑖𝑂 + 2𝐶𝑙− ↔ 𝑇𝑖𝐶𝑙2 + 𝑂2− 𝑝𝑂2− =

∆𝐺0

𝑅𝑇𝑙𝑛10

References 147

𝑇𝑖3𝑂2 + 6𝐶𝑙− − 2𝑒− ↔ 3𝑇𝑖𝐶𝑙2 + 2𝑂

2− 𝐸 =∆𝐺0

2𝐹−2𝑅𝑇𝑙𝑛10

2𝐹𝑝𝑂2−

𝑇𝑖4+ + 𝑒− ↔ 𝑇𝑖3+ 𝐸 =−∆𝐺0

𝐹

𝑇𝑖3+ + 𝑒− ↔ 𝑇𝑖2+ 𝐸 =−∆𝐺0

𝐹

𝑇𝑖2+ + 2𝑒− ↔ 𝑇𝑖 𝐸 =−∆𝐺0

2𝐹

𝑇𝑖𝑂2 + 4𝐶𝑙− ↔ 𝑇𝑖𝐶𝑙4 + 2𝑂

2− 𝑝𝑂2− =∆𝐺0

2𝑅𝑇𝑙𝑛10

𝑇𝑖𝑂2 + 3𝐶𝑙− + 𝑒− ↔ 𝑇𝑖𝐶𝑙3 + 2𝑂

2− 𝐸 =−∆𝐺0


𝐹𝑝𝑂2−

References 148

References

[1] IAEA, Nuclear Energy Development in the 21st Century: Global Scenarios and

Regional Trends, IAEA Nuclear Energy Series, Vienna, 2010.

[2] IEA, OECD, Key World Energy Statistics, Paris, 2014.

[3] IAEA, Nuclear Safety and Security, IAEA Annual Report, Vienna, 2010.

[4] C. Poinssot, S. Bourg, N. Ouvrier, N. Combernoux, C. Rostaing, M. Vargas-

Gonzalez, J. Bruno, Assessment of the Environmental Footprint of Nuclear Energy

Systems. Comparison between Closed and Open Fuel Cycles, Energy, 69 (2014)

199-211.

[5] OECD, OECD Factbook: Economic, Environmental and Social Statistics, Paris,

2013.

[6] P.D. Wilson, The Nuclear Fuel Cycle: From Ore to Waste, Oxford University

Press, 1996.

[7] M. Benedict, T.H. Pigford, H.W. Levi, Nuclear Chemical Engineering, 2 ed.,

McGraw-Hill, 1981.

[8] IAEA, PRIS Database, Vienna, 2015.

[9] G.F. Hewitt, J.G. Collier, Introduction to Nuclear Power, Taylor & Francis,

2000.

References 149

[10] E.R. Irish, W.H. Reas, The PUREX Process - a Solvent Extraction

Reprocessing Method for Irradiated Uranium, Symposiua on the Reprocessing of

Irradiated Fuels Held at Brussels, 20 (1957) 25.

[11] K.L. Nash, G.J. Lumetta, Advanced Separation Techniques for Nuclear Fuel

Reprocessing and Radioactive Waste Treatment, Woodhead Publishing, 2011.

[12] IEA, OECD, Nuclear Energy Data, Paris, 2014.

[13] D. Magnusson, B. Christiansen, J.P. Glatz, R. Malmbeck, G. Modolo, D.

Serrano‐Purroy, C. Sorel, Demonstration of a TODGA Based Extraction Process

for the Partitioning of Minor Actinides from a PUREX Raffinate, Solvent

Extraction and Ion Exchange, 27 (2009) 26-35.

[14] J.J. Laidler, J.E. Battles, W.E. Miller, J.P. Ackerman, E.L. Carls, Development

of Pyroprocessing Technology, Progress in Nuclear Energy, 31 (1997) 131-140.

[15] NEA, Pyrochemical Separations in Nuclear Applications, (2004).

[16] A.I. Bhatt, H. Kinoshita, A.L. Koster, I. May, C.A. Sharrad, V.A. Volkovich,

O.D. Fox, C.J. Jones, B.G. Lewin, J.M. Charnock, Actinide, Lanthanide and

Fission Product Speciation and Electrochemistry in High and Low Temperature

Ionic Melts, Centre for Radiochemistry Research, Department of Chemistry,

University of Manchester, Oxford Road, Manchester, M13 9PL (United Kingdom),

2004.

[17] Oak Ridge National Laboratory, Advanced Head-End Processing of Spent

Fuel: A Progress Report, ANS Annual Meeting, (2005).

[18] A. Landsberg, C.L. Hoatson, F.E. Block, The Chlorination Kinetics of

Zirconium Dioxide in the Presence of Carbon and Carbon Monoxide, Metallurgical

Transactions, 3 (1972) 521-527.

References 150

[19] M. Iizuka, T. Koyama, N. Kondo, R. Fujita, H. Tanaka, Actinides Recovery

from Molten Salt/Liquid Metal System by Electrochemical Methods, Journal of

Nuclear Materials, 247 (1997) 183-190.

[20] T. Koyama, M. Iizuka, N. Kondo, R. Fujita, H. Tanaka, Electrodeposition of

Uranium in Stirred Liquid Cadmium Cathode, Journal of Nuclear Materials, 247

(1997) 227-231.

[21] S.A. Kuznetsov, M. Gaune-Escard, Redox Electrochemistry and Formal

Standard Redox Potentials of the Eu(III)/Eu(II) Redox Couple in an Equimolar

Mixture of Molten NaCl-KCl, Electrochimica Acta, 46 (2001) 1101-1111.

[22] P. Masset, R.J.M. Konings, R. Malmbeck, J. Serp, J.-P. Glatz,

Thermochemical Properties of Lanthanides (Ln = La, Nd) and Actinides (An = U,

Np, Pu, Am) in the Molten LiCl–KCl Eutectic, Journal of Nuclear Materials, 344

(2005) 173-179.

[23] D. Lambertin, J. Lacquement, S. Sanchez, G.S. Picard, Americium Chemical

Properties in Molten LiCl–KCl Eutectic at 743 K, Plasmas & Ions, 3 (2000) 65-72.

[24] D. Lambertin, J. Lacquement, S. Sanchez, G. Picard, Determination of the

Solubility Product of Plutonium Sesquioxide in the NaCl+CaCl2 Eutectic and

Calculation of a Potential–pO2−

Diagram, Electrochemistry Communications, 4

(2002) 447-450.

[25] A.V. Bychkov, O.V. Skiba, S.K. Vavilov, M.V. Kormilitzyn, A.G. Osipenco,

Overview of RIAR Activity on Pyroprocess Development and Application to

Oxide Fuel and Plans in the Coming Decade, Pyrochemical Separations OECD

Workshop Proceedings, (2000) 37-46.

[26] M. Gaune-Escard, Molten Salts: From Fundamentals to Applications, Kluwer

Academic, 2002.

References 151

[27] D.R. Sadoway, New Opportunities for Metals Extraction and Waste Treatment

by Electrochemical Processing in Molten Salts, Journal of Materials Research, 10

(1995) 487.

[28] D. Inman, S.H. White, The Production of Refractory Metals by the

Electrolysis of Molten Salts; Design Factors and Limitations, Journal of Applied

Electrochemistry, 8 (1978) 375.

[29] B. Mishra, D.L. Olson, Molten Salt Applications in Materials Processing,

Journal of Physics and Chemistry of Solids, 66 (2005) 396.

[30] Y. Ito, T. Nohira, Non-conventional Electrolytes for Electrochemical

Applications, Electrochimica Acta, 45 (2000) 2611.

[31] G.J. Janz, Molten Salts Handbook, Academic Press, 1967.

[32] D. Sadoway, The Electrochemical Processing of Refractory Metals, Journal of

the Minerals, Metals and Materials Society, 43 (1991) 15-19.

[33] M. Galopin, J.S. Daniel, Molten Salts in Metal Treating: Present Uses and

Future Trends, Electrodeposition and Surface Treatment, 3 (1975) 1-31.

[34] D.J. Fray, Emerging Molten Salt Technologies for Metals Production, Journal

of the Minerals, Metals and Materials Society, 53 (2001) 27.

[35] B. Trémillon, Reactions in Solution: An Applied Analytical Approach, John

Wiley & Sons, 1997.

[36] L. Brunet, J. Caillard, P. André, Thermodynamic Calculation of n-Component

Eutectic Mixtures, International Journal of Modern Physics C, 15 (2004) 675-687.

[37] E. Van Artsdalen, I. Yaffe, Electrical Conductance and Density of Molten Salt

Systems: KCl–LiCl, KCl–NaCl and KCl–KI, The Journal of Physical Chemistry,

59 (1955) 118-127.

References 152

[38] H. Ito, Y. Hasegawa, Densities of Eutectic Mixtures of Molten Alkali

Chlorides below 673 K, Journal of Chemical & Engineering Data, 46 (2001) 1203-

1205.

[39] G.Z. Chen, D.J. Fray, T.W. Farthing, Direct Electrochemical Reduction of

Titanium Dioxide to Titanium in Molten Calcium Chloride, Nature, 407 (2000)

361-364.

[40] S.J. Oosthuizen, In Search of Low Cost Titanium: The Fray Farthing Chen

(FFC) Cambridge Process, The Journal of the South African Institute of Mining

and Metallurgy, 111 (2011) 1-6.

[41] W. Kroll, The Production of Ductile Titanium, Transactions of the

Electrochemical Society, 78 (1940) 35.

[42] A.J. Fenn, G. Cooley, D.J. Fray, Exploiting the FFC Cambridge Process,

Advanced Materials and Processes, 162 (2004) 51-53.

[43] Z. Chen, T.W. Farthing, D.J. Fray, Removal of Oxygen from Metal Oxides

and Solid Solutions by Electrolysis in a Fused Salt, Google Patents, 1999.

[44] R.O. Suzuki, Direct Reduction Processes for Titanium Oxide in Molten Salt,

Journal of the Minerals, Metals and Materials Society , 59 (2007) 68.

[45] R.O. Suzuki, S. Inoue, Calciothermic Reduction of Titanium Oxide in Molten

CaCl2, Metallurgical and Materials Transactions B, 34 (2003) 277.

[46] R. Suzuki, K. Ono, K. Teranuma, Calciothermic Reduction of Titanium Oxide

and in-situ Electrolysis in Molten CaCl2, Metallurgical and Materials Transactions

B, 34 (2003) 287-295.

[47] C. Schwandt, D.J. Fray, Determination of the Kinetic Pathway in the

Electrochemical Reduction of Titanium Dioxide in Molten Calcium Chloride,

Electrochimica Acta, 51 (2005) 66-76.

References 153

[48] K. Dring, Electrochemical Reduction of Titanium Dioxide in Molten Calcium

Chloride, Ph.D. Dissertation, Imperial College London, London, 2005.

[49] K. Dring, R. Dashwood, D. Inman, Voltammetry of Titanium Dioxide in

Molten Calcium Chloride at 900 °C, Journal of the Electrochemical Society, 152

(2005) E104-E113.

[50] R. Bhagat, D. Dye, S.L. Raghunathan, R.J. Talling, D. Inman, B.K. Jackson,

K.K. Rao, R.J. Dashwood, In-situ Synchrotron Diffraction of the Electrochemical

Reduction Pathway of TiO2, Acta Materialia, 58 (2010) 5057-5062.

[51] C. Schwandt, D.T.L. Alexander, D.J. Fray, The Electro-deoxidation of Porous

Titanium Dioxide Precursors in Molten Calcium Chloride under Cathodic Potential

Control, Electrochimica Acta, 54 (2009) 3819-3829.

[52] D.T.L. Alexander, C. Schwandt, D.J. Fray, The Electro-deoxidation of Dense

Titanium Dioxide Precursors in Molten Calcium Chloride giving a New Reaction

Pathway, Electrochimica Acta, 56 (2011) 3286-3295.

[53] D. Hu, W. Xiao, G. Chen, Near-net-shape Production of Hollow Titanium

Alloy Components via Electrochemical Reduction of Metal Oxide Precursors in

Molten Salts, Metallurgical and Materials Transactions B, 44 (2013) 272-282.

[54] M. Ma, D. Wang, W. Wang, X. Hu, X. Jin, G.Z. Chen, Extraction of Titanium

from Different Titania Precursors by the FFC Cambridge Process, Journal of

Alloys and Compounds, 420 (2006) 37.

[55] B. Jackson, M. Jackson, D. Dye, D. Inman, R. Dashwood, Production of NiTi

via the FFC Cambridge Process, Journal of the Electrochemical Society, 155

(2008) E171-E177.

[56] B.K. Jackson, The production of NiTi via the Fray Farthing Chen Cambridge

Process, Ph.D. Dissertation, Imperial College London, London, 2009.

References 154

[57] B. Jackson, D. Dye, D. Inman, R. Bhagat, R. Talling, S. Raghunathan, M.

Jackson, R. Dashwood, Characterisation of the FFC Cambridge Process for NiTi

Production using In-situ X-ray Synchrotron Diffraction, Journal of the

Electrochemical Society, 157 (2010) E57-E63.

[58] R. Bhagat, M. Jackson, D. Inman, R. Dashwood, The Production of Ti–Mo

Alloys from Mixed Oxide Precursors via the FFC Cambridge Process, Journal of

the Electrochemical Society, 155 (2008) E63.

[59] R. Bhagat, M. Jackson, D. Inman, R. Dashwood, Production of Ti–W Alloys

from Mixed Oxide Precursors via the FFC Cambridge Process, Journal of the

Electrochemical Society, 156 (2009) E1.

[60] K.S. Mohandas, D.J. Fray, FFC Cambridge Process and Removal of Oxygen

from Metal-Oxygen Systems by Molten Salt Electrolysis: An Overview,

Transactions of the Indian Institute of Metals, 57 (2004) 579.

[61] G. Chen, E. Gordo, D. Fray, Direct Electrolytic Preparation of Chromium

Powder, Metallurgical and Materials Transactions B, 35 (2004) 223-233.

[62] Y. Deng, Wang, W. Xiao, Jin, Hu, G.Z. Chen, Electrochemistry at

Conductor/Insulator/Electrolyte Three-Phase Interlines: A Thin Layer Model, The

Journal of Physical Chemistry B, 109 (2005) 14043-14051.

[63] P. Kar, J.W. Evans, A Model for the Electrochemical Reduction of Metal

Oxides in Molten Salt Electrolytes, Electrochimica Acta, 54 (2008) 835-843.

[64] K. Jiang, X. Hu, M. Ma, D. Wang, G. Qiu, X. Jin, G.Z. Chen,

“Perovskitization”‐Assisted Electrochemical Reduction of Solid TiO2 in Molten

CaCl2, Angewandte Chemie International Edition, 45 (2005) 428-432.

[65] G. Garcia-Belmonte, J. Garcia Belmonte, Bisquert, Impedance Analysis of

Galvanostatically Synthesized Polypyrrole Films. Correlation of Ionic Diffusion

and Capacitance Parameters with the Electrode Morphology, Electrochimica Acta,

47 (2002) 4263-4272.

References 155

[66] G. Garcia Belmonte, V. Garciabelmonte, J. Vikhrenko, J. Garciacanadas,

Bisquert, Interpretation of Variations of Jump Diffusion Coefficient of Lithium

Intercalated into Amorphous WO3 Electrochromic Films, Solid State Ionics, 170

(2004) 123-127.

[67] E. Krasicka-Cydzik, Deoxidation of Titania Foams, ECS Transactions, 50

(2013) 39-44.

[68] M.A. Henderson, Mechanism for the Bulk-Assisted Reoxidation of Ion

Sputtered TiO2 Surfaces: Diffusion of Oxygen to the Surface or Titanium to the

bulk?, Surface Science, 343 (1995) L1156-L1160.

[69] M.A. Henderson, A Surface Perspective on Self-Diffusion in Rutile TiO2,

Surface Science, 419 (1999) 174-187.

[70] R. Abdulaziz, L.D. Brown, D. Inman, S. Simons, P.R. Shearing, D.J.L. Brett,

Novel Fluidised Cathode Approach for the Electrochemical Reduction of Tungsten

Oxide in Molten LiCl–KCl Eutectic, Electrochemistry Communications, 41 (2014)

44-46.

[71] J.N. Hiddleston, A.F. Douglas, Fluidized Bed Electrodes - Fundamental

Measurements and Implications, Nature, 218 (1968) 601-602.

[72] J.R. Backhurst, J.M. Coulson, F. Goodridge, R.E. Plimley, M. Fleischmann, A

Preliminary Investigation of Fluidized Bed Electrodes, Journal of the

Electrochemical Society, 116 (1969) 1600-1607.

[73] T. Berent, I. Fells, R. Mason, Fluidized Bed Fuel Cell Electrodes, Nature, 223

(1969) 1054-1055.

[74] T. Berent, R. Mason, I. Fells, Fluidised-Bed Fuel-Cell Electrodes, Journal of

Applied Chemistry and Biotechnology, 21 (1971) 71-76.

References 156

[75] M. Fleischmann, J.W. Oldfield, L. Tennakoon, Fluidized Bed Electrodes Part

IV. Electrodeposition of Copper in a Fluidized Bed of Copper-Coated Spheres,

Journal of Applied Electrochemistry, 1 (1971) 103-112.

[76] L.J.J. Janssen, On Oxygen Reduction at a Fluidized Bed Electrode,


[77] A.J. Monhemius, P.L.N. Costa, Interactions of Variables in the Fluidised-Bed

Electrowinning of Copper, Hydrometallurgy, 1 (1975) 183-203.

[78] D. Hutin, F. Coeuret, Experimental Study of Copper Deposition in a Fluidized

Bed Electrode, Journal of Applied Electrochemistry, 7 (1977) 463-471.

[79] B.J. Sabacky, J.W. Evans, Electrodeposition of Metals in Fluidized Bed

Electrodes: Part II . An Experimental Investigation of Copper Electrodeposition at

High Current Density, Journal of the Electrochemical Society, 126 (1979) 1180-

1187.

[80] F. Coeuret, The Fluidized Bed Electrode for the Continuous Recovery of

Metals, Journal of Applied Electrochemistry, 10 (1980) 687-696.

[81] C. Oloman, A.P. Watkinson, Hydrogen Peroxide Production in Trickle-Bed

Electrochemical Reactors, Journal of Applied Electrochemistry, 9 (1979) 117-123.

[82] Y. Xiong, H.T. Karlsson, An Experimental Investigation of Chemical Oxygen

Demand Removal from the Wastewater Containing Oxalic Acid using Three-Phase

Three-Dimensional Electrode Reactor, Advances in Environmental Research, 7

(2002) 139-145.

[83] S. Germain, F. Goodridge, Copper Deposition in a Fluidised Bed Cell,


[84] F. Goodridge, C.J.H. King, A.R. Wright, Performance Studies on a Bipolar

Fluidised Bed Electrode, Electrochimica Acta, 22 (1977) 1087-1091.

References 157

[85] Y. Matsuno, A. Tsutsumi, K. Yoshida, Characteristics of Three-Dimensional

Electrodes for a Molten Carbonate Fuel Cell Anode, International Journal of

Hydrogen Energy, 20 (1995) 601-605.

[86] A.C. Lee, S. Li, R.E. Mitchell, T.M. Gür, Conversion of Solid Carbonaceous

Fuels in a Fluidized Bed Fuel Cell, Electrochemical and Solid-State Letters, 11

(2008) B20-B23.

[87] S. Li, A.C. Lee, R.E. Mitchell, T.M. Gür, Direct Carbon Conversion in a

Helium Fluidized Bed Fuel Cell, Solid State Ionics, 179 (2008) 1549-1552.

[88] J. Zhang, Z. Zhong, D. Shen, J. Xiao, Z. Fu, H. Zhang, J. Zhao, W. Li, M.

Yang, Characteristics of a Fluidized Bed Electrode for a Direct Carbon Fuel Cell

Anode, Journal of Power Sources, 196 (2011) 3054-3059.

[89] Y. Matsuno, K. Suzawa, A. Tsutsumi, K. Yoshida, Characteristics of Three-

Phase Fluidized-Bed Electrodes for an Alkaline Fuel Cell Cathode, International

Journal of Hydrogen Energy, 21 (1996) 195-199.

[90] Y. Matsuno, A. Tsutsumi, K. Yoshida, Improvement in Electrode Performance

of Three-Phase Fluidized-Bed Electrodes for an Alkaline Fuel Cell Cathode,

International Journal of Hydrogen Energy, 22 (1997) 615-620.

[91] J. Grimm, D. Bessarabov, R. Sanderson, Review of Electro-Assisted Methods

for Water Purification, Desalination, 115 (1998) 285-294.

[92] L. Neužil, F. Procháska, V. Bejček, M. Mocek, Modelling a Two-Stage

Countercurrent Fluidized Bed Reactor for Removing SO2 from Gases by Active

Soda, Computers & Chemical Engineering, 12 (1988) 205-208.

[93] F. Goodridge, C.J. Vance, Copper Deposition in a Pilot-Plant-Scale Fluidized

Bed Cell, Electrochimica Acta, 24 (1979) 1237-1242.

References 158

[94] K. Jüttner, U. Galla, H. Schmieder, Electrochemical Approaches to

Environmental Problems in the Process Industry, Electrochimica Acta, 45 (2000)

2575-2594.

[95] M. Fleischmann, F. Goodridge, J.R. Backhurst, 1194181, British Patent, 1970.

[96] W. Xiao, X. Xiao, H. Wang, H. Yin, X. Zhu, D. Mao, Wang, Verification and

Implications of the Dissolution–Electrodeposition Process During the Electro-

Reduction of Solid Silica in Molten CaCl2, RSC Advances, 2 (2012) 7588.

[97] W. Xiao, X. Xiao, G. Jin, Chen, Up-Scalable and Controllable Electrolytic

Production of Photo-Responsive Nanostructured Silicon, Journal of Materials

Chemistry A: Materials for Energy and Sustainability, 1 (2013) 10243.

[98] A. Boika, S.N. Thorgaard, A.J. Bard, Monitoring the Electrophoretic

Migration and Adsorption of Single Insulating Nanoparticles at

Ultramicroelectrodes, The Journal of Physical Chemistry B, 117 (2012) 4371-4380.

[99] D. Inman, G.J. Hills, L. Young, J.O.M. Bockris, Some Thermodynamic

Aspects of Molten Salts: Halides of Uranium, Zirconium, Thorium, and Cerium in

Alkali Halide Eutectics*, Annals of the New York Academy of Sciences, 79 (1960)

803-829.

[100] S. Kuznetsov, H. Hayashi, K. Minato, M. Gaune-Escard, Electrochemical

Transient Techniques for Determination of Uranium and Rare-Earth Metal

Separation Coefficients in Molten Salts, Electrochimica Acta, 51 (2006) 2463-

2470.

[101] J. Laidler, J. Battles, W. Miller, J. Ackerman, E. Carls, Development of

Pyroprocessing Technology, Progress in Nuclear Energy, 31 (1997) 131-140.

[102] J. Willit, W. Miller, J. Battles, Electrorefining of Uranium and Plutonium - A

Literature Review, Journal of Nuclear Materials, 195 (1992) 229-249.

References 159

[103] Z. Tomczuk, J.P. Ackerman, R.D. Wolson, W.E. Miller, Uranium Transport

to Solid Electrodes in Pyrochemical Reprocessing of Nuclear Fuel, Journal of The


[104] T. Koyama, M. Iizuka, Y. Shoji, R. Fujita, H. Tanaka, T. Kobayashi, M.

Tokiwai, An Experimental Study of Molten Salt Electrorefining of Uranium Using

Solid Iron Cathode and Liquid Cadmium Cathode for Development of

Pyrometallurgical Reprocessing, Journal of Nuclear Science and Technology, 34

(1997) 384-393.

[105] Y. Sakamura, T. Hijikata, K. Kinoshita, T. Inoue, T.S. Storvick, C.L.

Krueger, L.F. Grantham, S.P. Fusselman, D.L. Grimmett, J.J. Roy, Separation of

Actinides from Rare Earth Elements by Electrorefining in LiC1-KC1 Eutectic Salt,

Journal of Nuclear Science and Technology, 35 (1998) 49-59.

[106] J. Roy, L. Grantham, D. Grimmett, S. Fusselman, C. Krueger, T. Storvick, T.

Inoue, Y. Sakamura, N. Takahashi, Thermodynamic Properties of U, Np, Pu, and

Am in Molten LiCl‐KCl Eutectic and Liquid Cadmium, Journal of the


[107] J.A. Plambeck, Encyclopedia of Electrochemistry of the Elements: Fused

Salt Systems, Dekker, 1976.

[108] J.E. Kelly, Generation IV International Forum: A decade of Progress

Through International Cooperation, Progress in Nuclear Energy, 77 (2014) 240-

246.

[109] S. Herrmann, S. Li, M. Simpson, S. Phongikaroon, Electrolytic Reduction of

Spent Nuclear Oxide Fuel as Part of an Integral Process to Separate and Recover

Actinides from Fission Products, Separation Science and Technology, 41 (2006)

1965-1983.

[110] E.-Y. Choi, J.-M. Hur, I.-K. Choi, S.G. Kwon, D.-S. Kang, S.S. Hong, H.-S.

Shin, M.A. Yoo, S.M. Jeong, Electrochemical Reduction of Porous 17 kg Uranium

References 160

Oxide Pellets by Selection of an Optimal Cathode/Anode Surface Area Ratio,

Journal of Nuclear Materials, 418 (2011) 87-92.

[111] C.S. Seo, S.B. Park, B.H. Park, K.J. Jung, S.W. Park, S.H. Kim,

Electrochemical Study on the Reduction Mechanism of Uranium Oxide in a LiCl-

Li2O Molten Salt, Journal of Nuclear Science and Technology, 43 (2006) 587-595.

[112] S.M. Jeong, S.B. Park, S.S. Hong, C.S. Seo, S.W. Park, Electrolytic

Production of Metallic Uranium from U3O8 in a 20 kg Batch Scale Reactor, Journal

of Radioanalytical and Nuclear Chemistry, 268 (2006) 349-356.

[113] Y. Sakamura, M. Kurata, T. Inoue, Electrochemical Reduction of UO2 in

Molten CaCl2 or LiCl, Journal of The Electrochemical Society, 153 (2006) D31-

D39.

[114] J.-M. Hur, S.-S. Hong, H. Lee, Electrochemical Reduction of UO2 to U in a

LiCl–KCl-Li2O Molten Salt, Journal of Radioanalytical and Nuclear Chemistry,

295 (2013) 851-854.

[115] D. Inman, J.O.M. Bockris, The Application of the Galvanostatic Potential-

Time Technique to Analysis in Molten Salts, Journal of Electroanalytical

Chemistry (1959), 3 (1962) 126-145.

[116] A. Baraka, A.I. Abdel-Rohman, A.A.E. Hosary, Corrosion of Mild Steel in

Molten Sodium Nitrate-Potassium Nitrate Eutectic, British Corrosion Journal, 11

(1976) 44-46.

[117] R.W. Bradshaw, N.P. Siegel, Molten Nitrate Salt Development for Thermal

Energy Storage in Parabolic Trough Solar Power Systems, Proceedings of Energy

Sustainability, (2008) 10-14.

[118] K. Coscia, S. Neti, A. Oztekin, S. Nelle, S. Mohapatra, T. Elliott, The

Thermophysical Properties of the NaNO3-KNO3, LiNO3-NaNO3, and LiNO3-KNO3

Systems, Proceedings of the ASME 2011 International Mechanical Engineering

Congress and Exposition, Denveer, Colorado, (2011).

References 161

[119] D.A. Nissen, Thermophysical Properties of the Equimolar Mixture Sodium

Nitrate-Potassium Nitrate from 300 to 600 °C, Journal of Chemical & Engineering

Data, 27 (1982) 269-273.

[120] D.J. Rogers, G.J. Janz, Melting-Crystallization and Premelting Properties of

Sodium Nitrate-Potassium Nitrate. Enthalpies and heat capacities, Journal of

Chemical & Engineering Data, 27 (1982) 424-428.

[121] J.O.M. Bockris, G.J. Hills, D. Inman, L. Young, An All-Glass Reference

Electrode for Molten Salt Systems, Journal of Scientific Instruments (1950), 33

(1956) 438-439.

[122] G. Qiu, M. Ma, D. Wang, X. Jin, X. Hu, G.Z. Chen, Metallic Cavity

Electrodes for Investigation of Powders Electrochemical Reduction of NiO and

Cr2O3 Powders in Molten CaCl2, Journal of the Electrochemical Society, 152

(2005) E328-E336.

[123] G. Qiu, X. Feng, M. Liu, W. Tan, F. Liu, Investigation on Electrochemical

Reduction Process of Nb2O5 Powder in Molten CaCl2 with Metallic Cavity

Electrode, Electrochimica Acta, 53 (2008) 4074-4081.

[124] J. Peng, Y. Deng, D. Wang, X. Jin, G.Z. Chen, Cyclic Voltammetry of

Electroactive and Insulative Compounds in Solid State: A Revisit of AgCl in

Aqueous Solutions Assisted by Metallic Cavity Electrode and Chemically

Modified Electrode, Journal of Electroanalytical Chemistry, 627 (2009) 28-40.

[125] J. Peng, G. Li, H. Chen, D. Wang, X. Jin, G.Z. Chen, Cyclic Voltammetry of

ZrO2 Powder in the Metallic Cavity Electrode in Molten CaCl2, Journal of the

Electrochemical Society, 157 (2010) F1-F9.

[126] G. Qiu, D. Wang, X. Jin, G.Z. Chen, A Direct Electrochemical Route from

Oxide Precursors to the Terbium–Nickel Intermetallic Compound TbNi5,


References 162

[127] V.N. Baumer, S.S. Galkin, L.V. Glushkova, T.P. Rebrova, Z.V. Shtitelman,

Solubility of Al2O3 in Some Chloride−Fluoride Melts, Inorganic Chemistry, 45

(2006) 7367-7371.

[128] A. Levin’sh, M. Straumanis, K. Karlsons, Praezisionsbestimmung von

Gitterkonstanten Hygroskopischer Verbindungen (LiCl, NaBr), Zeitschrift für

Physikalische Chemie. Abteilung B Chemie der Elementarprozesse, Aufbau der

Materie, 40 (1938) 146-150.

[129] R. Littlewood, Diagrammatic Representation of the Thermodynamics of

Metal‐Fused Chloride Systems, Journal of the Electrochemical Society, 109 (1962)

525.

[130] M. Pourbaix, Thermodynamics of Dilute Aqueous Solutions, E. Arnold,

London, 1949.

[131] K. Dring, R. Dashwood, D. Inman, Predominance Diagrams for

Electrochemical Reduction of Titanium Oxides in Molten CaCl2, Journal of the

Electrochemical Society, 152 (2005) D184.

[132] L.D. Brown, R. Abdulaziz, S. Simons, D. Inman, D.J.L. Brett, P.R. Shearing,

Predominance Diagrams of Uranium and Plutonium Species in Both Lithium

Chloride–Potassium Chloride Eutectic and Calcium Chloride, Journal of Applied

Electrochemistry, 43 (2013) 1235-1241.

[133] D. Inman, N.S. Wrench, Corrosion in Fused Salts, British Corrosion Journal,

1 (1966) 246.

[134] A. Conte, M.D. Ingram, Corrosion of Silver in Fused Nitrates: Applications

of E/pO2-

Diagrams, Electrochimica Acta, 13 (1968) 1551.

References 163

[135] G. Picard, F. Seon, B. Trémillon, Reactions of Formation and Stability of

Iron (II) and (III) Oxides in LiCl‐KCl Eutectic Melt at 470 °C, Journal of the

Electrochemical Society, 129 (1982) 1450.

[136] T.H. Okabe, Y. Waseda, Producing Titanium Through an Electronically

Mediated Reaction, Journal of the Minerals, Metals and Materials Society, 49

(1997) 28.

[137] A.M. Martı́nez, Y. Castrillejo, E. Barrado, G.M. Haarberg, G. Picard, A

Chemical and Electrochemical Study of Titanium Ions in the Molten Equimolar

CaCl2 + NaCl Mixture at 550 °C, Journal of Electroanalytical Chemistry, 449

(1998) 67.

[138] T. Abiko, I. Park, T.H. Okabe, Reduction of Titanium Oxide in Molten Salt

Medium, Ti-2003 10th World Conference on Titanium, Hamburg, 2003, 253.

[139] C. Caravaca, A. Laplace, J. Vermeulen, J. Lacquement, Determination of the

E-pO2-

Stability Diagram of Plutonium in the Molten LiCl–KCl Eutectic at 450 °C,

Journal of Nuclear Materials, 377 (2008) 340.

[140] X.Y. Yan, D.J. Fray, Direct Electrolytic Reduction of Solid Alumina Using

Molten Calcium Chloride-Alkali Chloride Electrolytes, Journal of Applied

Electrochemistry, 39 (2009) 1349.

[141] S. Wang, Y. Li, Reaction Mechanism of Direct Electro-Reduction of

Titanium Dioxide in Molten Calcium Chloride, Journal of Electroanalytical

Chemistry, 571 (2004) 37-42.

[142] NEA, Spent Nuclear Fuel Assay Data for Isotopic Validation State-of-the-

Art Report, (2011).

References 164

[143] G. Radulescu, D.E. Mueller, J.C. Wagner, Sensitivity and Uncertainty

Analysis of Commercial Reactor Criticals for Burnup Credit, U.S. Nuclear

Regulatory Commision, (2008).

[144] V. Smolenski, A. Smolenski, J. Laplace, Lacquement, A Potentiometric

Study of the Interaction of Zr(IV) and O(II) Ions in the LiCl-KCl Eutectic Molten

Salt, Journal of the Electrochemical Society, 151 (2004) E302.

[145] I. Barin, Thermodynamical Data of Pure Substances, VCH Verlags

Gesellschaft, Weinheim, 1993.

[146] M.W. Chase, JANAF thermochemical tables, American Institute of Physics,

New York, 1985.

[147] Landolt-Bornstein, Thermodynamic Properties of Inorganic Materials,

Springer-Verlag, Berlin-Heidelberg, 1999.

[148] V. Glushko, Thermocenter of the Russian Academy of Sciences, IVTAN

Association, Izhorskaya, 13 (1994) 127412.





[151] O. Knacke, O. Kubaschewski, K. Hesselmann, Thermodynamic Properties of

Inorganic Substances, Springer Verlag, Düsseldorf, Germany, (1991) 1.

[152] M. Binnewies, E. Milke, Thermochemical data of elements and compounds,

Wiley-VCH, 2002.

References 165

[153] D.G. Archera, Thermodynamic Properties of the KCl+ H2O System, Journal

of Physical and Chemical Reference Data, 28 (1999).



[155] NEA, R. Guillaumont, F.J. Mompean, Update on the Chemical

Thermodynamics of Uranium, Neptunium, Plutonium, Americium and

Technetium., Elsevier, 2003.

[156] L.P. Ruzinov, B.S. Gulyanitskii, Equilibrium Transformations of

Metallurgical Reactions, Moscow, 1975.





[159] A.J. Bard, R. Parsons, J. Jordan, Standard Potentials in Aqueous Solution,

Taylor & Francis, 1985.

[160] M.K. Karapet'iants, M.L. Karapet'iants, Thermodynamic constants of

inorganic and organic compounds, Humphrey Science Publishers, 1970.

[161] J.R. Haas, E.L. Shock, D.C. Sassani, Rare Earth Elements in Hydrothermal

Systems: Estimates of Standard Partial Molal Thermodynamic Properties of

Aqueous Complexes of the Rare Earth Elements at High Pressures and

Temperatures, Geochimica et Cosmochimica Acta, 59 (1995) 4329-4350.

References 166

[162] L.B. Pankratz, M. United States. Bureau of, Thermodynamic Properties of

Carbides, Nitrides, and Other Selected Substances, U.S. Department of the Interior,

U.S. Bureau of Mines, Washington, D.C.

[163] M.W. Chase, NIST-JANAF thermochemical tables, New York, 1998.

[164] C. Rostaing, C. Poinssot, D. Warin, P. Baron, B. Lorraina, Development and

Validation of the EXAm Separation Process for Single Am Recycling, Procedia

Chemistry, 7 (2012) 367-373.

[165] E. Lassner, W. D. Schubert, Tungsten: Properties, Chemistry, Technology of

the Element, Alloys, and Chemical Compounds, Springer Science & Business

Media, 2012.

[166] E. Lassner, From Tungsten Concentrates and Scrap to Highly Pure

Ammonium Paratungstate (APT), The Chemistry of Non-Sag Tungsten,

Pergamon, Oxford, 1995, 35-44.

[167] V. Malyshev, A. Gab, A. M. Popescu, V. Constantin, Electroreduction of

Tungsten Oxide(VI) in Molten Salts with Added Metaphosphate, Chemical

Research in Chinese Universities, 29 (2013) 771-775.

[168] X. Xi, G. Si, Z. Nie, L. Ma, Electrochemical Behavior of Tungsten Ions from

WC Scrap Dissolution in a Chloride Melt, Electrochimica Acta, 184 (2015) 233-

238.

[169] G. Si, X. Xi, Z. Nie, L. Zhang, L. Ma, Preparation and Characterization of

Tungsten Nanopowders from WC Scrap in Molten Salts, International Journal of

Refractory Metals and Hard Materials, 54 (2016) 422-426.

References 167

[170] K. Dring, R. Dring, M. Bhagat, R. Jackson, D. Dashwood, Inman, Direct

Electrochemical Production of Ti–10W Alloys from Mixed Oxide Preform

Precursors, Journal of Alloys and Compounds, 419 (2006) 103-109.

[171] R. Diehl, G. Brandt, E. Salje, The Crystal Structure of Triclinic WO3, Acta

Crystallographica Section B: Structural Crystallography and Crystal Chemistry, 34

(1978) 1105-1111.

[172] M. Rosenthal, P. Kasten, R. Briggs, Molten-Salt Reactors—History, Status,

and Potential, Nuclear Technology, 8 (1970) 107-117.

[173] M.J. Earle, J.M. Esperança, M.A. Gilea, J.N.C. Lopes, L.P. Rebelo, J.W.

Magee, K.R. Seddon, J.A. Widegren, The Distillation and Volatility of Ionic

Liquids, Nature, 439 (2006) 831-834.

[174] H. Hartmann, F. Ebert, O. Bretschneider, Elektrolysen in

Phosphatschmelzen. I. Die Elektrolytische Gewinnung von α‐und β‐Wolfram,

Zeitschrift für Anorganische und Allgemeine Chemie, 198 (1931) 116-140.

[175] N. Rees, Y.G. Rees, R. Zhou, Compton, The Aggregation of Silver

Nanoparticles in Aqueous Solution Investigated via Anodic Particle Coulometry,

ChemPhysChem, 12 (2011) 1645-1647.

[176] E.J.E. Stuart, N.V. Rees, R.G. Compton, Particle-Impact Voltammetry: The

Reduction of Hydrogen Peroxide at Silver Nanoparticles Impacting a Carbon

Electrode, Chemical Physics Letters, 531 (2012) 94-97.

[177] N. Rees, Y.G. Rees, R. Zhou, Compton, Making Contact: Charge Transfer

during Particle–Electrode Collisions, RSC Advances, 2 (2012) 379.

[178] K. Tschulik, B. Haddou, D. Omanović, N. Rees, R. Compton, Coulometric

Sizing of Nanoparticles: Cathodic and Anodic Impact Experiments Open Two

References 168

Independent Routes to Electrochemical Sizing of Fe3O4 Nanoparticles, Nano

Research, 6 (2013) 836-841.

[179] M. Williamson, J. Willit, Pyroprocessing Flowsheets for Recycling Used

Nuclear Fuel, Nuclear Engineering and Technology, 43 (2011) 329-334.

[180] S. Barrett, A. Jacobson, B. Tofield, B. Fender, The Preparation and Structure

of Barium Uranium Oxide BaUO3+x, Acta Crystallographica Section B: Structural

Crystallography and Crystal Chemistry, 38 (1982) 2775-2781.

[181] I. Uchida, J. Niikura, S. Toshima, Electrochemical Study of UO22+

-UO2+-

UO2 System in Molten LiCl+KCl Eutectic, Journal of Electroanalytical Chemistry

and Interfacial Electrochemistry, 124 (1981) 165-177.

[182] L. Brown, R. Abdulaziz, R. Jervis, V. Bharath, R. Attwood, C. Reinhard, L.

Connor, S. Simons, D. Inman, D. Brett, Following the Electroreduction of Uranium

Dioxide to Uranium in LiCl–KCl Eutectic In Situ using Synchrotron Radiation,


[183] S. Abramowitz, N. Acquista, K.R. Thompson, Infrared Spectrum of Matrix-

Isolated Uranium Monoxide, The Journal of Physical Chemistry, 75 (1971) 2283-

2285.

[184] M.C. Heaven, J.P. Nicolai, S.J. Riley, E.K. Parks, Rotationally Resolved

Electronic Spectra for Uranium Monoxide, Chemical Physics Letters, 119 (1985)

229-233.

[185] M.H. Mueller, R.L. Hitterman, H.W. Knott, The Atomic Position Parameter

in Alpha Uranium–Room Temperature and Above, Acta Crystallographica, 15

(1962) 421-422.

References 169

[186] P.R. Shearing, J. Golbert, R.J. Chater, N.P. Brandon, 3D Reconstruction of

SOFC Anodes using a Focused Ion Beam Lift-Out Technique, Chemical

Engineering Science, 64 (2009) 3928-3933.

[187] P.R. Shearing, R.S. Bradley, J. Gelb, S.N. Lee, A. Atkinson, P.J. Withers,

N.P. Brandon, Using Synchrotron X-Ray Nano-CT to Characterize SOFC

Electrode Microstructures in Three-Dimensions at Operating Temperature,

Electrochemical and Solid-State Letters, 14 (2011) B117-B120.

[188] P. Shearing, R. Bradley, J. Gelb, F. Tariq, P. Withers, N. Brandon, Exploring

Microstructural Changes Associated with Oxidation in Ni–YSZ SOFC Electrodes

using High Resolution X-ray Computed Tomography, Solid State Ionics, 216

(2012) 69-72.

[189] R. Clague, P. Shearing, P. Lee, Z. Zhang, D. Brett, A. Marquis, N. Brandon,

Stress Analysis of Solid Oxide Fuel Cell Anode Microstructure Reconstructed from

Focused Ion Beam Tomography, Journal of Power Sources, 196 (2011) 9018-9021.

[190] P. Trogadas, O.O. Taiwo, B. Tjaden, T.P. Neville, S. Yun, J. Parrondo, V.

Ramani, M.O. Coppens, D.J. Brett, P.R. Shearing, X-ray Micro-Tomography as a

Diagnostic Tool for the Electrode Degradation in Vanadium Redox Flow Batteries,

Electrochemistry Communications, 48 (2014) 155-159.

[191] D.S. Eastwood, P.M. Bayley, H.J. Chang, O.O. Taiwo, J. Vila-Comamala,

D.J. Brett, C. Rau, P.J. Withers, P.R. Shearing, C.P. Grey, Three-Dimensional

Characterization of Electrodeposited Lithium Microstructures using Synchrotron

X-ray Phase Contrast Imaging, Chemical Communications, 51 (2015) 266-268.

[192] D.J.L. Brett, A. Atkinson, N.P. Brandon, S.J. Skinner, Intermediate

Temperature Solid Oxide Fuel Cells, Chemical Society Reviews, 37 (2008) 1568-

1578.

References 170

[193] W. Maskell, B. Steele, Solid State Potentiometric Oxygen Gas Sensors,

Journal of Applied Electrochemistry, 16 (1986) 475-489.

[194] D.K. Corrigan, E.O. Blair, J.G. Terry, A.J. Walton, A.R. Mount, Enhanced

Electroanalysis in Lithium Potassium Eutectic (LKE) Using Microfabricated

Square Microelectrodes, Analytical Chemistry, 86 (2014) 11342-11348.

[195] E.O. Blair, D.K. Corrigan, J.G. Terry, A.R. Mount, A.J. Walton,

Development and Optimization of Durable Microelectrodes for Quantitative

Electroanalysis in Molten Salt, Journal of Microelectromechanical Systems, 24

(2015) 1346-1354.

[196] J.J. Powers, B.D. Wirth, A Review of TRISO Fuel Performance Models,


[197] S. Langer, N. Baldwin, H. Phillips, Head-End Separation of Triso-Coated

Fissile and Fertile Particles for High-Temperature Gas-Cooled Reactors, Nuclear

Technology, 12 (1971) 26-30.

[198] C. FitzqerakJ, V. Vaughen, K. Notz, R. Lowrie, Head-End Reprocessing

Studies with Irradiated HTGR-Type Fuels: III. Studies With RTE-7: TRISO UC2-

TRISO ThC2, (1975).

[199] G. Del Cui, C. Forsberg, W. Rickman, TRISO-Coated Fuel Processing to

Support High-Temperature Gas-Cooled Reactors, ORNL, 27 (2002) 4-00.

[200] A. Radkowsky, A. Galperin, The Nonproliferative Light Water Thorium

Reactor: A New Approach to Light Water Reactor Core Technology, Nuclear

Technology, 124 (1998) 215-222.

[201] P.R. Kasten, Review of the Radkowsky Thorium Reactor Concept, Science

& Global Security, 7 (1998) 237-269.

References 171

[202] R. Rainey, J. Moore, Laboratory Development of the Acid Thorex Process

for Recovery of Thorium Reactor Fuel, Nuclear Science and Engineering, 10

(1961) 367-371.

[203] R. Rainey, J. Moore, Laboratory Development of the Acid Thorex Process

for Recovery of Consolidated Edison Thorium Reactor Fuel, Oak Ridge National

Laboratory, Tennessee, 1962.

[204] A. Nuttin, D. Heuer, A. Billebaud, R. Brissot, C. Le Brun, E. Liatard, J.M.

Loiseaux, L. Mathieu, O. Meplan, E. Merle-Lucotte, Potential of Thorium Molten

Salt Reactors Detailed Calculations and Concept Evolution with a View to Large

Scale Energy Production, Progress in Nuclear Energy, 46 (2005) 77-99.

[205] L. Mathieu, D. Heuer, R. Brissot, C. Garzenne, C. Le Brun, D. Lecarpentier,

E. Liatard, J.-M. Loiseaux, O. Meplan, E. Merle-Lucotte, The Thorium Molten Salt

Reactor: Moving on from the MSBR, Progress in Nuclear Energy, 48 (2006) 664-

679.

[206] L. Mathieu, D. Heuer, E. Merle-Lucotte, R. Brissot, C. Le Brun, E. Liatard,

J.-M. Loiseaux, O. Maplan, A. Nuttin, D. Lecarpentier, Possible Configurations for

the Thorium Molten Salt Reactor and Advantages of the Fast Nonmoderated

Version, Nuclear Science and Engineering, 161 (2009) 78-89.

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